I agree full heatedly with both Mario Party and Portal 2... especially when I tired playing Portal 2 co-op with both my friend, my cousin, and my mother... *shutters*
Other than that, I would personally add Naruto Ultimate Ninja Storm 3 to the list especially if your friend's been playing the game longer than you... You would think it would be simple (O button to start a combo --> Triangle to end combo in jutsu/technique --> Charge while opponent is down --> Rinse and Repeat), but considering the matches take place on a full 3D field and your opponent knows how to navigate the field better than you because, again, they've played the game longer than you, good luck hitting them with a single combo, considering there's also button to avoid most combos... I would rather play any of the Mario Party games at this point... Plus, honorable mention to Call of Duty-based multiplayer where perks come from killing your opponent than from your opponent killing you...