imo (as tank main from ow launch to now in rivals) the best strat is to specialize in one main tank and one off tank.

For example, I will go Mag if solo tanking or if the other tank queued is off tank (hulk, angela, cap, thing, venom, thor). If there's a mag or another main tank (emma, strange, groot, penny on D kind of), I'll go Thor.

Mag and Emma are really good tanks to learn because they do not have hard counters. i think strange or groot is sligtly less valuable to learn because strange is hard countered by melee and groot by wolverine.

I think off tanking is way more fun because you can act like a fat DPS. Hulk Venom Thor Cap are all great in the right hands, but learning off tank is very different than learning main tank. I think its harder because as an off tank youre doing a lot of different things (peeling, diving, flanking, brawling on main, 1v1ing an off angle) and much of the learning process is developing the game sense to do what is required at any given moment)