The game of "War" is the easiest card game to play.
So what did I win?
Seriously now, other than war I think 5-card stud poker is one of the easiest games to play.
Five cards in your hand, make your bet if you think you have the best hand and it's over, unless there's a raise involved, then you call or fold.
Other than 5 card stud poker I think blackjack is fairly simple too.