This page lists my opinions on Species, Backgrounds and Gods tiers.
Tier 1: Broken
DD > Ce
Deep Dwarf: Strong melee that can heal at will, which is ridiculously overpowered, and has very high HP.
Centaur: Faster than most enemies so can kite enemies with melee and all types of offensive magic and exploit bows, and has high HP.
Tier 2: Strong
Tr > Mi > Gn > Sp > HO > Dr
Troll: Extremely high HP, and extremely strong unarmed combat offense capabilities. Also fast regen, as if trolls weren't already super strong.
Minotaur: All-around melee/ranged race with excellent aptitudes for not-magic. Retaliatory headbutt makes D:1 fairly free. High HP.
Gnoll: Melee starts have the highest starting HP in-game; mage starts start able to cast every spell in their book. Can do everything, and without any opportunity cost.
Spriggan: Faster than almost all enemies, however its -30% HP means it can be one-shot on D:1 on a non-melee start. Once it reaches XL 2 it is excellent.
Hill Orc: Excellent melee race that falls short of Mi because of the lack of an auxiliary attack and worse aptitudes. High HP.
Draconian: No encumbrance but high innate AC makes them excellent hybrids/mages, and their XL 7 breath weapon takes them through a large chunk of the early game. AC+4 and +10% HP on D:1 mean it makes tier 1.
Tier 3: Average to strong
Og > Mf > Gr > Vp > Fe > Gh > Hu
Ogre: +30% HP but aptitudes designed to make you cry, and generally the worst defenses in the game. This makes them painful to play and not as strong as they look. Still, +30% HP and +3 fighting skill is enough to put them at the top of tier 3.
Merfolk: Their +4 polearms aptitude means they are excellent as soon as they find a polearm. They have poor defenses though due to a -3 armour apt, and common polearms struggle to keep up with damage; their weird aptitudes and average HP mean they fall down to Mi and HO.
Gargoyle: -20% HP gives them a weak D:3-7, however at that point their large bonuses to AC and innate resistances make them good conjurers and the highest-defense melees in-game.
Vampire: Almost completely vanilla XL 1-2, but at XL 3 they get bat form, allowing them spriggan-speed, and their weird hunger system allows them access to rPois.
Felid: All mage starts can get one-shot by a dart slug at XL 1 on entering LOS, and all starts can be one-shot in melee. This is the only reason they are not in tier 2. At XL 2 they get enough HP not to be one-shot, and their fast speed and then extra lives make up for this massively. Extremely tedious though due to -40% HP, so nobody ever wants to play them.
Ghoul: Humanoid with worse aptitudes but high HP and claws 1, and rC+ and poison immunity. Claws 1 isn't very good on D:1, but the high UC apt makes it worthwhile later.
Human: The baseline, but somewhat underrated. They make very good everythings, because they have above average aptitudes (yes, 0 is above average in almost everything) - also it turns out I rate it number 15, so maybe that says something not about the lacking power of Hu but that the gimmicks of other races are better.
Tier 4: Average to weak
Dg > Te > Ds > Fo > Ha > VS
Demigod: Overrated. Slightly worse than human overall: they have a better early game with +10% HP and better stats, but they fall down later on when every other race has a god. And gods are really good. And so without gods Dg basically has no good panic buttons except the standard ones. +10% MP is trivial, and I would prefer a god to HP and stats past XL 6.
Tengu: -20% HP makes them weak until XL 5 - auxiliary attacks are redeeming - when they get fast temporary flight so should never again die to 10-speed melee enemies. Well above average aptitudes, but low HP and AC makes them weak in melee.
Demonspawn: Poor aptitudes make them weak until they get their first mutations, at which point they get increasingly strong. They are unreliable though, and still are hindered by flat -1s.
Formicid: Really strong melees once they get a shield and a two-handed weapon, but fairly weak in melee early on, with average HP. Their stasis means they lose access to 3 of the most valuable consumables: blinking, haste and teleportation, and this means fear, as unreliable as it is, is their best escape. Digging does not relevantly make up for this.
Halfling: +4 slings aptitude makes them quite strong once they find a sling on D:1-3, but weak until then due to -10% HP - although they are small, which is helpful. Requires Oka/Trog for slings to work forever - and that makes them very significantly worse.
Vine Stalker: Poor aptitudes, but -30% HP offset by spirit shield and very fast regen that makes them pretty capable at melee and hybridisation. However their XL 1-3 is one of the worst in the game, and hence they are weak.
Tier 5: Weak
Op > Ba > DE > Ko
Octopode: No AC is a massive problem, but as partial compensation they get a constriction melee attack. Their aptitudes are flexible and there are ways of killing things from range, but when enemies enter melee things can get very rough - and most of the time those enemies are going to be fast enemies. They get destroyed by centaurs. (I do think that putting Op among Na and Mu is an insult to octopodes, however.)
Barachi: Slow, which is absolutely terrible. They would be bottom tier if it wasn't for hop, which lets them fight with relative safety only near stairs. Notably they have the highest aptitudes overall, which makes them good hybrids and specialists alike.
Deep Elf: Best magic aptitudes - but worst melee aptitudes, awful strength and -20% HP means a non-pure mage start is horrible for them, and they rely on finding good spells as they go through the game. The -2 fighting apt hurts here.
Kobold: Almost completely just worse DE. They are better AM, En, Hu and As, but worse everything else. High EV and carnivorous make them ok mages, but once again horrible melees, made worse by small limiting their weapons.
Tier 6: Terrible
Na > Mu
Naga: Slowest, which is horrific. Starts with 1 AC (except Fi/Gl/As). Poison spit, rPois and high HP make them not the worst race in the game, but these things are not infallible - however they become strong later on.
Mummy: 3 of the biggest problems: the worst aptitudes, rF- and no potions. If they don't find rF++ by the time they need to go to vaults they are in trouble. Gozag fixes their worst issue with access to potions, but even still they can very easily get wrecked by orc wizards.
Tier 1: Excellent
Gozag > Nemelex > Kikubaaqudgha > Okawaru > Ru > Hepliaklqana > Yredelemnul > Trog
Gozag: He gives access to an immediate burst of potion effects as a panic button as soon as you convert. This continues to be exceptional throughout the game, along with the access he gives to equipment, and bribe branch which, if used carefully, can make areas, including Zot:5, free.
Nemelex: He gives you a deck of cards almost instantly upon worship, which is guaranteed to have 4 cards which help you deal with nasty situations. Once you have a few decks, you have enough cards to use in every dubious situation Nemelex throws your way. Later on, Deal Four on any deck of summoning is going to instantly remove the threat from your situation, and on better decks, deal with every enemy you need to.
Kikubaaqudgha: At 1* you get a book, and Receive Corpses. This lets you spam the corpse-using spell(s) in your book, which is strong. At 3* you get spells that take you through the game, and if you're lucky enough to get Simulacrum (about 70% chance I think) you can win the game with only his spells and corpse delivery. Hence Kiku is
Okawaru: Super fast piety gain gives you access to heroism very rapidly, which is a huge increase in power and lets you deal with most enemies. Also the only god to properly facilitate ranged, and because ranged is strong, so is Okawaru.
Ru: Sacrifices can be rough early on, but once you reach 3*, you become super strong. Her abilities only get better - power leap functions as a 3 tile controlled blink, and apocalypse is both a panic button and ridiculously powerful.
Hepliaklqana: Gives an ally as soon as you start worship, and that is always worth the frailty, because allies are really strong. You can sacrifice your ancestor if necessary, and fight with it to boost your damage. Contrary to popular belief, knight is slightly stronger than hexer, and both of these are very significantly better than the battlemage, in my opinion. This is because you want a tanky ally that can take damage for you.
Yredelemnul: Upon reaching 1* of piety, you only have to kill about three enemies on a floor to win that floor. On reaching 3*, you are pretty much carried to depths by his allies which are incredibly strong. You also get drain life, which is an excellent panic button with more than about 2 enemies visible. Unfortunately he has the problem that you still have to kill those 3-4 enemies in the early game, and that can be an issue.
Trog: Lack of magic is a relevant problem, but as soon as you get a weapon and 1*, you have some of the strongest god abilities, notably trog's hand means you never have to worry about MR again, and BiA's allies are absurd.
Tier 2: Good
Jiyva > Makhleb > Elyvilon > Beogh > Fedhas > Zin > Lugonu
Jiyva: Fastest piety gain in the game. Rarely an option, but a good option - her only problems are losing items to slimes and requiring 4* for slimify (but realistically you should be at 4* immediately after worship), which is Usk's grand finale but doesn't require invo or a fight beforehand. You also get good mutations and better stats as a bonus.
Makhleb: Suffers slightly from not doing much until 2*. At 2* you get minor destruction which almost nobody uses even though it's free and really good. Later on you get allies as well - Makhleb will solve all your problems at 3* for a very long time.
Elyvilon: Piety gain is fast and from exploration. Upon reaching 2*, you have a way to deal with all living enemies provided you have about 10 invo, in heal other. You also get free curing potions. Elyvilon's 4* and 5* abilities are also amazing - +HP and MP is ridiculous and so is heal wounds.
Beogh: Does absolutely nothing until 2*, which takes a while, and smite is not spammable. Can only be worshipped by HO, and its strength makes Beogh a non-issue. Smite is good, allies are good, but the lack of abilities before 2* is a problem.
Fedhas: The most tedious god, and should be removed. The only abilities that do very much are evolution and growth, and even though wandering mushrooms are fairly strong, you take 3 times as long to do anything because positioning is a nightmare. Fedhas also suffered heavily from the food changes recently, making his later power completely dependent on turncount and spellcasting.
Zin: Recite doesn't deal with what you need it to deal with. Vitalisation is also mediocre. It's not until imprison at 3* that this god becomes good, but it then becomes very good. Imprison is a great panic button, and sanctuary is the best in the game. You are removed from the mutation game, which is helpful.
Lugonu: Does nothing until 2*, and bend space is almost a worse Hep transference that is unreliable. Banish, corrupt, and enter the abyss are ridiculously strong abilities (offense and escape), but you can't use them as much as you want because you don't have the piety.
Tier 3: Mediocre
Wu Jian > Uskayaw > Ashenzari > Sif Muna > Dithmenos > Vehumet > the Shining One
Wu Jian: The martial attacks themselves are mediocre - the only remarkable thing is whirlwind pin allowing for a guaranteed 1 tile gap for escaping, which is why it isn't even lower on this list. At 3* you get a worse Ru power leap after roughly the same amount of time - it does slightly more damage, is slightly worse as an escape option, and also gives Exh. And Serpent's Lash is by far the best thing about Wu Jian; the noise on Heavenly Storm makes it a bad ability.
Uskayaw: The strength is line pass, which does something immediately. The weakness is that he doesn't actually do very much - the paralysis doesn't last, pain bond is exciting but not necessary, and by the time you can use grand finale you don't need it - it is such a niche case.
Ashenzari: Instead of giving more freedom early on, Ash gives less freedom by requiring curses. His skill boost is not irrelevant later on - notably this makes Ash competitive for gnolls. Scrying is very helpful in certain situations, but it isn't a make-or-break ability.
Sif Muna: Does something at 1* which is better than Vehumet. That something still really isn't significant though, however channeling at 3* is some attempt at an emergency ability - it isn't very good, but it's better than nothing. Her book gifts come far too late in my opinion - if they were at 2* or 3* Sif would be much stronger.
Dithmenos: Fairly good god for a stealthy character, not much good otherwise. Your umbra starts at 1* and shadow step at 2*, which complement a stabber very well. It's not until shadow mimic at 4* that he does anything non-stabber, and recently it was made that this is completely rubbish anyway on anything except spells. Shadow form is a pretty good escape ability, but a 5* ability isn't enough to pull Dithmenos out of tier 3.
Vehumet: Vehumet reliably gives you the conjurations spells to make it through the game. However, you're only likely to get a worthwhile gift on the third gift, which takes horrifically long. Even worse is the lack of any active abilities, and so Veh falls down to even Sif.
The Shining One: A very good option for melees in extended, but gods aren't ranked by their performance in extended. Divine shield and allies are good, but your piety gain is painful - you struggle to make it to 2* even, and only at 4* does he do anything significant. SDW can be compared to Trog BiA, but SDW is going to take half the game to get to.
Tier 4: Detrimental
Qazlal > Cheibriados > Xom
Qazlal: Bombards you with enemies because of the noise. I believe his abilities are good, and if played carefully is very fun, but the noise makes Qazlal a poor god before you get upheaval at 2*.
Cheibriados: Gods are supposed to provide escape options; instead Chei takes the cheapest and most reliable one, walking, away (let alone haste), and immediate bend time comes nowhere near making up for this. Until you get step from time at 5* and you can use it, you are basically in the hands of your enemies. This god can make mincemeat of even the best races.
Xom: Can just kill you. Not sure that the potential for banishment after 20 turns of picking him up needs much explanation.