For me an insane mon that always magically gets on my team is Arbok. The early evolution along with intimidate for pivoting make it run through a lot of the early game, but in my playthrough it has been SO good in boss fights. I've been running something like Glare, Acid Spray, Screech, Sludge bomb to just debuff big threaths.
Probably not underrated but I've been loving using the new dragon Leavanny. It's design is soooo sick, but I've been running Fell Stinger for the boost, and then Baton Passing it onto another teammate for some cool combos,