I have played through both games now and I wanted to try to contribute to this a little. I have never posted in an in-game tier list thread for real before but I guess you must start somewhere. I am not sure if the rules I play with are the ones that are the optimal ones when ranking Pokemon for these threads (I play with Exp. Share off, I max the affection of all my Pokemon in Refresh and I only use Z-moves in important battles) so keep that in mind. I have understood that trading over stones to evolve certain Pokemon early is not accepted for these lists, so I will not rank the Pokemon for which I did that.
Here are the Pokemon I used:
Ultra Moon
Litten
Final moveset: Flare Blitz / Darkest Lariat / Brick Break / Flamethrower
Available early with good stats all-round, the only lacking one is Speed. It's movepool was good too, the only real problem was that it learned Flare Blitz very late. I decided to not go with Fire Punch which meant I had to go with Fire Fang for a long while. Darkest Lariat was a great secondary STAB move, then I went with Brick Break for coverage and Flamethrower in the last slot because I couldn't think of anything better. It was one of the better members of the team, it felt like it helped in many important battles and I had great success with it. Not sure what else to say, the Litten line is just really great.
A
Abra (with trade)
Final moveset: Psychic / Shadow Ball / Dazzling Gleam / Energy Ball
Available early, could potentially be a bit of a pain to catch but I managed to catch the first Abra I met in the first Ultra Ball I threw. It needed a bit of babying before it evolved into Kadabra but it became really strong afterwards. It then got even stronger once it had evolved into Alakazam, I traded to evolve mine after beating Hala but I guess the optimal time to fully evolve it would be right after it has evolved into Kadabra. Stat-wise, it has really high Sp.att and Speed as well as a pretty good movepool. I went with Recover instead of Energy Ball for a long while but scrapped Recover late in the game since I rarely used it. The bad thing is that Alakazam has terrible defenses, especially on the physical side. In the late stages of the game, if it couldn't KO the opponent, it would very likely get KO'd in return. Overall, it was quite good but not the best.
B
Petilil
Final moveset: Petal Dance / Giga Drain / Quiver Dance / Sleep Powder
Not going to rank this since I traded over a Sun Stone to evolve it early, before it would learn Quiver Dance as a Lilligant. That said, if the Sun Stone had been available earlier in the game, I think it would be A for sure. Quiver Dance was the recipe for success it needed to break the game in half. Lilligant was easily the best member of my Ultra Moon team, it took care of many situations where the others couldn't do much. Even without Quiver Dance, I can see it working decently with Giga Drain, Sleep Powder, Growth/Z-Growth and Petal Dance+Own Tempo. Probably B or C, but as said, I won't rank it.
Lillipup
Final moveset: Return / Crunch / Rock Tomb / Giga Impact
Available almost directly on Akala Island, so early mid-game. Good Attack, decent Speed and bulk, Intimidate was also helpful to lower the Attack of opposing Pokemon. Sadly, it's movepool was a bit lacking. It takes a long while for the Return TM to be available, so I was stuck with Take Down for a large chunk of the game. I used Facade alongside Take Down for a while, it doesn't cause recoil and it is more accurate, but comes at the cost of being a bit weaker. Regarding other moves, the only useful one it gets through level-up is Crunch. There's also Play Rough but it comes very late. The same goes for Giga Impact but I think it is better to just use the TM. I used it to get the most powerful Z-move that was possible. I also used Work Up to raise its Attack for a short while but I scrapped it as I found it better to just attack right away. Rock Tomb was the final move, it was surprisingly useful. Lowering the Speed of opposing Pokemon is never bad. Overall, I find Stoutland to be quite good but it was not without issues.
B/C
Mantyke
Final moveset: Scald (Surf after beating the game) / Air Slash / Ice Beam / Roost
I backtracked to Melemele to catch one after getting the ability to Surf on Lapras (so I would have it before the Fire Trial), though I'm not sure if that is the best moment to catch one. It also requires a Remoraid to evolve but finding one is not hard since you already have the Fishing Rod at that point. As for Mantine, it has insane Sp.def while its other stats are a bit lackluster. It is slow and has lacking offenses. Like most Water-types, it also takes forever before it gets a good Water-type move. Air Slash and Roost are cool, but they are not enough. Water Absorb was also fun since the AI was sometimes stupid enough to use Water-moves against Mantine. I won most battles with Mantine but the majority of them were slow victories since it didn't do much damage with its moves and had to heal itself with Roost throughout the battles. Still, its Special bulk should not be underestimated as it took a +4 Dazzling Gleam from Totem Ribombee like nothing. Mantine is not the best, but not completely useless either.
D
Mimikyu (regular, not the Totem version)
Final moveset: Play Rough / Shadow Claw / Leech Life / Hone Claws (Swords Dance after beating the game)
Available quite late, after beating the Ghost Trial. It also has a low chance of appearing but I got lucky and found one quickly. Unfortunately, it felt underwhelming as it rarely did enough damage. After I had beaten the game and unlocked the IV Judge, I found out that my Mimikyu had a
0 IV in Attack, that's probably one reason. It needed Hone Claws to do more damage, but by the time it had set up it still didn't feel like it did enough. Either that, or it had already fainted from the damage it took while setting up. Play Rough also had a tendency to miss at the worst possible moments if I didn't use Hone Claws to raise Mimikyu's Accuracy first. Leech Life could be good to recover HP but I rarely found use for it in the later stages of the game. Disguise is a great Ability, but it never felt like it helped all that much. I can see Totem Mimikyu being really good in these games, but sadly I didn't quite feel it with regular Mimikyu. I still love it though.
B
Ultra Sun
Buneary
Final moveset: Return / High Jump Kick / Fire Punch / Splash (Thunder Punch after beating the game and getting the Mega Stone)
Available very early, on Route 1. Mine evolved very quickly, not too long after the Normal Trial. Though I did use some Vitamins from Festival Plaza and a Friendship Power from Rotom to speed up the process. It gets Return upon evolving, which reaches full power soon afterwards. Sadly, Lopunny doesn't have very good stats. Z-Splash is pretty cool but it wasn't enough. Bulky opponents still survived a +3 move and it rarely felt worth setting up since many trainers in the game only have 1 Pokemon on their team. Lopunny is also really frail, so setting up was risky. That said, it usually survived more hits than I expected. Jump Kick rarely missed, which was another good thing. Overall an okay starter replacement and early team member but it is far from perfect.
C
Oricorio (not
since it is post-game only)
Final moveset: Revelation Dance / Air Slash / Hurricane / Roost
Probably the best member of my team on this game. It only has Flying-moves for the majority of the game but it doesn't need much more. Air Cutter/Air Slash and Roost was usually good enough. The ability to change form and type was interesting but never super useful since you can only change between there types during the game and it takes a while before you can change at all. Still, it was sometimes useful to change its type into something else before a boss battle. Revelation Dance was also good for secondary STAB in the later stages of the game. Very good overall, though I'm not sure if it is quite worthy of S rank.
A
Tentacool
Final moveset: Scald (Surf after beating the game) / Sludge Wave / Ice Beam / Giga Drain
Available after clearing the Water Trial, early mid-game. Stat-wise, it has good Sp.def and quite good Speed, but its other stats are lacking. I expected it to be at least somewhat decent, but it was not. It was utter garbage. It needed a lot of babying at first and it never really got better once it had caught up with the rest of the team in terms of level. It suffers from the same problem as most Water-types with the TMs for Scald and Surf being available too late, and it never really gets anything good before that either. Lowering opposing Pokemon's Sp.def with Acid Spray looks cool in theory, but it doesn't work in practice. Tentacool will either die in the time it takes to lower their Sp.def, or it will still not do enough damage afterwards. Giga Drain also looks great but it is too weak without STAB or a boost in some form. Icy Wind is a great way to lower opposing Pokemon's Speed for team support but it doesn't have much other uses. And once Tentacruel finally gets the better moves like Scald and Sludge Wave, they are still not doing enough damage. I hate to say anything positive about Tentacruel, but it had a few positive things going for it. Icy Wind was great for team support. Clear Body is also a good Ability since a lot of opponents will try to lower the stats of your Pokemon. It was also one of the two fastest members of the team. Overall, it felt like it worked the best mid-game, surprisingly. It just didn't work early- or late-game. Even during the post-game, it failed to OHKO a Mudsdale at 10 levels lower with Surf and got destroyed by Earthquake in return. I can't remember the last time I was this disappointed with a Pokemon I used on an in-game team. Looking back, I really regret not using another Water-type, or another Poison-type. Easily the worst Pokemon of this team, as well as the worst Pokemon I have used on my in-game teams across all four Gen 7 games. I will probably never use it again.
E
Eevee (Leafeon)
Final moveset: Leaf Blade / X-Scissor / Bite (Knock Off after beating the game) / Swords Dance
Definitely one of the better members of the team. It has great Attack and Defense, its Speed is also quite good. Its Sp.def is terrible though, I could never expect it to survive neutral or even resisted special moves. I'm not sure about the best point to catch an Eevee if you want to use Leafeon, I caught mine after completing the Grass Trial so I could evolve it immediately afterwards. It get Razor Leaf upon evolving, then Seed Bomb through the Ula'ula Move Tutor and then finally Leaf Blade later on, so it will always have a pretty strong STAB move. It has decent movepool but it is hard to know what to use. I went with the moves listed above but Synthesis and Aerial Ace are also alternatives. Leafeon is strong but its non-STAB moves apart from X-Scissor are quite weak and won't do much damage unless it boosts its Attack with Swords Dance first. Overall, it felt very useful at many times, I could count on it to be useful when other members of the team couldn't do much.
B
Pineco
Final moveset: Bug Bite / Heavy Slam / Rock Slide / Drill Run (Earthquake after beating the game)
Available early on Ula'ula Island. It has fantastic Defense and decent Attack but lower Sp.def and very low Speed. It evolves quickly after capture which is good. Bug Bite is unfortunately very weak for a main STAB move, and it is in an awkward position when it comes to Gyro Ball. It is too slow to outrun anything, but too fast for Gyro Ball to do any significant damage. Heavy Slam is better, but it becomes available very late. It does get Rock and Ground-type moves for coverage so that's at least something. I could always count on Forretress to survive any Physical hit, and even any Special hit thanks to Sturdy as long as it had full HP. Unfortunately, it could rarely KO anything back. Most opponents were 2-3HKO'd if Forretress survived long enough. Overall it was not the best, but definitely not the worst either.
D
Vulpix (Alolan)
Final moveset: Ice Beam / Dazzling Gleam / Psyshock / Nasty Plot
Like with Lilligant, I traded over an Ice Stone to evolve it early so I won't rank it. But unlike Lilligant, it didn't feel like the earlier Ice Stone helped that much. Vulpix was pathetically weak, Ninetales was a bit better but still lacking. It learns Ice Beam quickly after capture, then Dazzling Gleam upon evolving. Psyshock for coverage and then I used Calm Mind to set up, later replaced with Nasty Plot. Unfortunately, the Z-Crystals for both its STAB types are among the last ones to be found in the games. It would probably be D or C, but I won't rank it.
