Inteleon comes in very late (post-Adam), but makes up for it by being extremely efficient at killing things. He carried Saki hard for me - oneshotting half her team by himself after Wonder Room removal. 100% Crit Rate Sniper is one hell of a drug.
Aevium Parasect also was a major player, putting a stop to any glass cannon Pokemon (hi Geara's Mega Gengar).
Sandaconda is also surprisingly reliable - between Coil, Dig, Rock Tomb and Glare, she can set up on anything and hit back hard. My Sandaconda took out 2 of Ryland's most terrifying mons by herself - Excadrill and M-Camerupt. She outdid all her competition until I got a Garchomp.
Grumpig's also very useful early game - a lynchpin in the first 3 gyms, and one of the earliest ways to get Magic Coat to completely laugh at Crawli.
Perrserker was a powerhouse earlygame, and her usefulness only really waned at Erick.
Cooperajah is Perrserker's replacement, and boy he does not disappoint - oneshotting Gardevoir from 60% health is no small feat.
Alolan Ninetales came in clutch against Zetta/Geara, and hoo boy she does not stop being useful, even without Aurora Veil access.
Boltund really puts in work against Narcissa, Valerie and Souta - he hits hard and is very fast, though lack of Ice Fang does hurt. He's still the best boy though.
Electrode, Male Meowstic and Pyukumuku are what I'd call the cheese squad, able to cripple and tank many bosses that would be impossible otherwise (hi Rift Gardevoir).
And lastly, my starter, Rillaboom. Kind of weak before Rillaboom, but after evolving, hoo boy the pain train never stops. Wood Hammer hits everything hard, and he has a very varied movepool, with amazing hits like Knock Off and Acrobatics. Drum Beating is a very useful utility, too.
Edit: Forgot Greedent lmao. She's a goddamn monster and is basically diet Snorlax with inbuilt Counter, which is ridiculously good earlygame and still pulls her weight until post-Adam for me. 95 base Attack and 120 base HP before Keta is ludicrously broken imo.
I usually use a debuffing strategy against Rift Mons: Pyukumuku Purifies the field, removing it, then Toxic (If it's slow and bulky or to get rid of Lum Berries in the case of Rift Gardevoir) or Memento (if it's fast and hard-hitting). Electrode Paralyses and decreases the enemies Def and SpA if possible, then set up a screen, then Meowstic come in and decrease the Rift's Atk and SpD and set up screens if Electrode couldn't or the Rift is a mixed attacker. After this, the Rift should be mostly neutered so I can send out my hard hitters to break the shield and kill the Rift.