I'd say that Alex or MC would win. Mario I can't exactly see winning, Kratos doesn't know what the hell poker is, and while Dante has poker skills and I am a fanboy (yes I admit that, but you should've already known from my name), he absolutely sucks at poker.
Alex on the other hand, has thousands of minds and every one of them most likely have experience with poker.
MC... actually it's hard to say if he's good or not, but with his helmet on, he has the ultimate poker face.
Kratos loses the first round and attempts to beat everyone to death with the table.
Dante loses next because he sucks at gambling.
All that's left is Mercer, Mario, and Master Chief.
Mercer gets disqualified for cheating (regenerating clothes) and Chief wins for having armor and possible clothing inside.