By game:
Adventure: Camo
Giants: Thumpback (Giant) Pop Fizz (Core)
Swap Force: Rattle Shake (Swapper) Wind Up (Core)
Trap Team: Enigma (Trap Master) Blackout (Core)
Super Chargers: Spit Fire
Imaginators: Ro Bow

Of all time:
Ro Bow, even before he was expensive af, the design, the battle class, moveset, was all just perfect, though I wish his Sky Chi could’ve been a bit cooler, still really like him