|Your bank account is going to hate this movie as much as you and/or your kids will love it. :P|
Hulk: Barbarian, cursed to transform into a monster when the berserker rage overtakes him.
Thor: Fighter. A god can be a fighter, right? He's rather miffed about having his prized magic hammer destroyed.
Valkyrie: Paladin. After all, she works in service to a deity, who presumably granted her special powers and abilities. I'm not well-versed in Norse mythology, so I might be wrong. Still, paladin feels right.
Loki: Some sort of rogue class. He's a sneaky-pete, has a preference for short bladed weapons like knives and daggers, and uses deception and misdirection.
It's an impressive party for sure, but having said that, I find the lack of clerics, spellcasters, and ranged weapons worrisome.