Well that would depend on the point you are at in the game, which bits of the game you prefer and what I actually can level up due to covers.
Silver surfer, old man Logan, Phoenix, hulk buster, iceman. Some might put Jean grey here but if I have 5*s I don't really need her as a deterrent.
High end realistic
Hulk buster, Jean grey, iron fist, any two others. Probably ice man and cyclops. But the first 3 rule the roost at the moment.
Prof X, scarlet witch, grey suit black widow, falcon, iceman.
So you don't have the covers and cash for them...
Patch, Loki, iron fist, cyclops, hulk
Patch and hulk for hulk bombs, patch and Loki for saving on health packs and cyclops with iron fist is one of the strongest 3 star combos.
OBW, wolverine, storm, magneto, cap
Black widow is the strongest 2 star, and wolvies strike tiles boost her passive to. Storm and magneto give you a stun combo. Caps shields help a lot in PvE
Iron man, black widow, juggernaught, nobody. Though you may be forced to level storm to have a usable character early