The main mechanics of theros are bestow, heroic, scry, devotion and monstrosity. only the last 2 are relevant for momir and its only monstrosity you have any control over.

Monstrosity is usually a good mechanic for momir just because there is nothing that can punish you for it, no bounce in response etc. Some groundpounders, ex nessian asp might not be worth leveling up since they can get chumpblocked easily.

8-drops: Ashen Rider. Very good and is another out to some sticky situations, so 8-drops is even more than before going to be the go-to drop for when you need removal.

Colossus of Akros: A very interesting card. Usually when you make 8-drops you are emptyhanded so if you want to leveö this guy up u have to wait 2 turns and make no drops. a 20/20 indestructyible trampler usually ends the game is 2-3 turns and he also defends from all groundpounders during these 2 turns. As usually, I would be more inclined to go for it if the board is stable. the drops your opponent makes during these 2 turns before he camn attack can bolster his defenses a lot though.

7-drops: As those who choose black at the theros prerelease knows, this guy makes at least 2 small fliers when he arrives, usually some more. Add to that fact that you usually have chaff from lower drops to toss to him, this seems like an excellent 7-drop.

6-drops: Evangel of heliod: Much much worse than the overlord since evasion is king in momir, and the fact that u dont have to sacc anything isnt really relevant since small groundounders don't do much in momir.

Horizon scholar. Basically a 4/4 vanilla flier, but are you looking for a specific land for some activated ability he can help you.

Benthic giant. hexproof isnt great in momir since there isnt that much removal, and hexproof on something without evasion is next to worthless since that is not what your opponent want to kill anyway.

Hythonia the cruel. An excellent card and is everything Kederekt Leviathan dreams of being, It can wipe the board and attack for 7 for any time you want. However i think youre almost always want to activate this at the end of your opponent turn so that you can untap and be the first one to make a drop in addition to your 7/9 deathtouch. Also remember that you can really slowroll her ability. One way to play her is to skip drops and add lands to your board, going for emrakul. Keep doing this until your opponent has lethal attackers or is killing her with something. If something goes wrong, you can usually jut blow her up and go from there. (She does not protect you for something like Bogardan hellkite when youre at 5 for example)

Shipbreaker kraken: Reminds me of Lorthos. Should win you most games it goes monstrous. I don't see how your opponent ever kills a 10/10 in combat when youre tapping their 4 best creatures so this should be able to swing freely too.

Medomai the ageless: Plays itself, taking extra turns are as broken in momir as almost everywhere else

Titan of eternal fire: There are a lot of humans in mtg, the fact that it does cost mana to ping is pretty restrictive though, downgrading drops just to ping some isnt usually where you wanna be, though sometimes it can be relevant.

Other 6.drops are a 6/5 trampler, a 5/5 with a very expensive monstrousability that doesnt grant evasion (and therefore pretty useless) and a 3/3 pseudo-pitchburn devils

For the rest of the drops im just going to discuss the most relevant ones.

Underworld cerberus: a really scary card and he attacks well in company with others, since if they want to gangblock him which they must if they want to block him at all, your other creatures usually get through.

Keepsake gorgon: If opponent doesnt have something too impressive out, slowroll his ability. unconditional removal is so rare in momit´r that you should be way way more careful with it than in regular limited.

Stormbreath dragon: Remember than you can activate him in your opponents drawstep, should he have something that stops it like ant queen out. ( Or if he just chumped it, making it unneccesary to activate monstrosity right away)

Stoneshock giant: Has a sick ability, which is somewhat hampered by flying creatures on both sides, but anyway; If you hit him try to avois trading creatures away if possible, and if your opponent hits him, try to trade as aggressively as possible.

Triad of fates: An excellent card in momir, since drawing extra card is less good in momir than in regular limited or constructed.

Ember swallower. activate monstrosity only if youre ahead, and remember that a 7/8 with no evasion is usually not going to change whos ahead on board or not.

Polukranos: His ability is really expensive, i would only use it if i could kill 2-3 dudes or some important evasiondudes.

The gods: the red one is the best, followed by the green one which is ok, but the white and black one is quite bad, they have pretty worthless abilitys. of course they all are decent if they are creatures, but that should not happen often. The blue one is reasonable, but you rarely have more than one island in play, due to it being the land people tend to play the least of.

Barnished hart: he accelerates you by 2 turns if u take a turn off to sacc him. that going to be worth it on nearly every boardstate, esoecially since you get a free chump with him the turn you take him.

Shipwreck singer: It reminds me of rage nimbus and its ability suffers from some of the same problems, its win-more. if your opp doesnt want to attack youre usually ahead anyways. treat it as a 1/2 flier most of the time.

For the lowest drops 0-3 there are quite  a lot of stinkers due to a lot of heroic, bestow and there are a 3manacreature that kills itself. There are are however 3 manaaccelerators in the 2dropslot (Sylvan caryatid, voaying satyr, satyr hedonist) so making 2drops on the play looks better than ever.