Just to start things off, I'm curious what people think look like the top cards in the format. I'll take a shot at some of my top picks, in rough order of power.
Rekindling Phoenix - this gets my vote for best card in the set. 4/3 flyer for 4 is already a very strong B+ imo, and your opponent basically MUST have removal for the token (or exile for the phoenix) or you get reanimation for FREE. It even gets HASTE for some insane reason. Without an answer this seems stone unbeatable, and even if they do it's a 4/3 flyer for 4 that forced removal (often twice). That's just insane.
Form of the Dinosaur - maybe there's a way to beat this card, but on the surface that seems extremely difficult. It puts you at a stable life total and kills a creature every single turn. Sure, you'll be taking damage to the face, but the board will soon be clear and you'll be able to beat face to close out the game. I'm sure it will backfire occasionally but it still seems completely bonkers.
The Immortal Sun - Pretty hard to lose if you untap with this. Anthem, mana, AND card advantage is pretty absurd in one package. There are games where it's too slow, but it's also colorless so it can go anywhere and I can't imagine passing it.
Ravenous Chupacabra - holy crap this card is completely insane. Best uncommon by a country mile, in a set with a lot of great uncommons. This beats nearly every rare and mythic out there, too. Necrataal was always insane, and that was back when terror and doom blade and such were the standard removal. Compared to other removal in ixalan, this card is stupidly good and will be a first pick in nearly every pack it exists in. If you ever get passed this, it's because no one who saw it was in black, period.
Tetzimoc, Primal Death - I think this is the best of the elder dinos in limited. Especially if you can afford 4BBB. A huge beater and a likely board wipe to boot. Potentially a little slow vs aggro but I think you'll be able to slip activations in on occasion, and potentially stabilize when it hits.
Tendershoot dryad - if you didn't already have the city's blessing, this will get it quickly, and then you'll have an army of 3/3s. Granted, it's fragile, and a little expensive, but if it lives for even a few turns, it seems pretty close to unbeatable.
Hadana's Climb - anyone remember citadel siege? Granted, this is 1/2 strength siege, and it's multicolor, but it costs less and it flips into a pretty powerful land (not always an upside, but you can probably prevent it flipping if you really want). Getting this out turn 3 behind a good 2 drop seems almost unbeatable to me.
Twilight Prophet - 2/4 flyer for 4 is a totally acceptable but not exciting card, so obviously most of the estimation here rides on how difficult the blessing of the city will be. As long as you can get it, you seem nearly unbeatable with this card on the field.
profane procession - this is a little lower because it's probably going to feel weak vs aggro, but in control mirrors it looks utterly unbeatable.
Azor, the Lawbringer - very color intensive, but certainly difficult to beat without hard removal, ideally not sorcery-based. If you're ahead it seems almost impossible to lose, if you're behind it seems like it could do a lot of work to catch you up. Cost is rather high, though.
Top remaining uncommons by color:
Everdawn Champion - best white uncommon. If your opponent can't remove it, they're pretty screwed on the ground, and the cost is super low. Excellent anti-aggro card.
Thrashing Brontodon - gotta be the best green uncommon. Awesome stats, and a very useful activated ability when it's necessary.
Reckless Rage - pinning down the best red uncommon is a little tricky, there are a lot of contenders but none that seem obviously the best. I'll throw in with reckless rage, though, it dishes out a lot of damage for a very low cost, even if you'll need a high-toughness (or doomed) creature to make it work best.
Siren Reaver - not the most exciting, but a 3/2 flyer for probably 3 is still the best blue uncommon.
Cleansing Ray - Looks quite Sideboardy. Even worse because the creature removal part is in the color of the tribe it hoses. Not an early pick by any stretch. Divine Verdict- Certainly maindeckable. Probably better for Vampires or Fliers than Dinosaurs given it can hit blockers. (Lets your tokens through.) Exultant Skymarcher - Better than a Wind Drake, so certainly has a spot. Not flashy, but it will get the job done. (Must check on how it stacks up to other Common fliers.) Legion Conquistador - Even better than it was in Ixalan, as the density is even higher and the Forerunner of the Legion can find it to kickstart the process. Also helps towards Ascend Luminous Bonds - More expensive Pacifism is still Pacifism. Not as good against Enrage, but likely excellent against Merfolk. Notably hard removal against pirates. Also counts towards City Blessing (but so does your opponent's card.) Martyr of Dusk - Solid. Trades and maintains permanent count for Ascend. Moment of Triumph - Nick trick. Life gain rider is more important than in most environments. Looks nice on Adanto Vanguard. Raptor Companion - Looks pretty okay for WR Dinosaur decks. Unfortunately dies to the Dino Forerunner. Sanguine Glorifier - More balanced version of 4/2 black equivalent from SOI. Likely slightly better (which is not saying a whole lot) given the token theme. Snubhorn Sentry - Looks good for Ascend decks if those work. Also notable as a 1-drop Dinosaur. Squire's Devotion - Likely better than Moment of Triumph. Immediate token (which allows it to chain though not the best play). Also, really good on Adanto Vanguard. Sun Sentinel - Slightly better bear is bear. Sun-Crested Pterodon - For dinos, 2/5 Flying Vigilance is a huge wall that can chip away at life totals. Looks like plant (NPI) for WG Dino deck.
Overall: Vampire stuff overall looks quite nice. Dinos are a bit more... all over there.
Crashing Tide - First card is best card (for Merfolk). Seriously, this is Repulse. Even Pirates don't mind this because they love tempo. Dead-Eye Rig-Hauler - What was that about Pirates and Tempo? Definite solid card. Kitesail Corsair - It's a 2/1 Flying on attack, which is an interesting take on Welkin Tern. Fairly high pick regardless. Mist-Cloaked Herald - 1/1 Unblockable for 1 is... well, a basis for a deck if you get all of the +1/+1 counter shenanigans going... Negate - Nice for either Merfolk or Pirates. Not very high pick, but something you probably want one of. River Darter - Seems situational. It seems very modal. Against Dinos it's a better Phantom Warrior, but against the others it's actually a pretty good blocker. Sailor of Means - I like these. Dino players also like seeing them (against them). Clearly meant by WotC to be a linchpin of Treasure decks. Sea Legs - Quite a nice card. Pretty flexible. Doesn't really help fragile Pirates against removal, though. Secrets of the Hidden City - Restrictive Divination with possible upside. May be a deck with ultra-cheap Merfolk vomiting their hand onto the battlefield and then playing this to restock. WU Vampire/Merfolk token decks would make this very nice. Soul of the Rapids - Rather expensive for 2-toughness. 3-power flyer that is hard to remove for 5 is still good, I guess. Spire Winder - 2/3 Flying for 4 is... not that impressive. There are a lot of cards that can block it. Ascending makes it quite a bit better, but this and Secrets are the only Blue Ascend Commons... perhaps top of curve for Merfolk Ascend decks. Sworn Guardian - Basic Merfolk is basic. Not likely to be played unless Merfolk Ascend is a thing. Waterknot - Claustrophobia reprint is always welcome. Like the Pacifism card in White, both counts for your Ascension and doesn't lower your opponent's count.
Overall: Possibly a fast Merfolk Ascension deck that refills with Secrets. Couple of very nice Tempo/Bounce cards.
Canal Monitor - Looks pretty forgettable. Not a Tribe, and 5/3 Vanilla for 5 is... meh. Dark Inquiry - Not too impressed, but has definite situational uses. Would prefer to discard 2 cards. Dinosaur Hunter - May help against random dinos. Still bear with random upside cannot be bad. Notable good target for Sea Legs. Dusk Charger - Er ok? Black Ascend decks likely will leverage White for Vampire tokens or Pirate colors for Treasures, but this is neither tribe. Don't think payoff is worth the risk of basic Hill Giant. Dusk Legion Zealot - Very solid. Unholy offspring of Phyrexian Rager and Elvish Visionary. Fathom Fleet Boarder - 3 for 3/3 in Black is always welcome. In Pirate deck, likely not to trigger downside. In Vampire deck, lifeloss may not be downside. Grasping Scoundrel - 2/1 Attacker for 1 in Black? Sure, why not. Very nice aggro Pirate. Note that early drops that aren't worthless help with Ascend. Gruesome Fate - Vampire token finisher. Impale - Typical Limited Black Removal. You're still playing it. Moment of Craving - Good removal. Notably kills Adanto Vanguard. Recover. Quite playable. Vampire Revenant. 3 power flier for 4 isn't terrible, but Vampires have a problem with 1 toughness. Voracious Vampire. Can help to push through if you can keep board clear.
Overall: Pirates seem to do better than Vampires in Black. Removal is always appreciated.
The ascend mechanic activates quite a bit more easily that I would have thought. The 3/3 black horse is no slouch. I got the beat down from the 0/3 1 cmc white dino once it ascended. It's among my first pics die sure.