Well for a two player game, I'd recommend Super Street Fighter IV. It's a great laugh to play once you get the hang of it.
Other than that, Crackdown is fun to play with a friend I hear, and it's dirt cheap too. Mass Effect is one of the most engrossing RPGs I've played, so that's a safe bet too.
And, finally, no matter what somebody asks or for what system, I recommend that you go out and buy Okami.