Here's a couple I've found pretty reliable in every fangame:
- Swalot: definitely on the tank side, access to poison and sleep thanks to yawn, gulpin and swalot got me through so much boss battles, they can usually tank 1 hit from almost everything and guarantee a status condition, definitely useful until level 55/60
- The rock. You know who I'm talking about, that 3 stage rock type Evo you usually get in random mountains/ volcanoes. May that be golem, gigalith or colossal, they usually are a great defensive wall and can bring in some heavy hits (also, some of them have access to sturdy, which is a plus)
- Speaking of rocks, Crustle. Great defense, sturdy and access to shell smash (which is guaranteed by sturdy) is a real game changer.
- Swinub is a bit hard to train, but mamoswine hits hard and has priority through ice shard.
- Meowstic. Prankster. That's enough right?
- Bronzong comes in handy later in the game, but is usually available pretty early. Great defensive Mon, with some tricks under its... Bell