Call of Duty 4: Modern Warfare
This might look like a patriotic game that will teach the kids American values, but it has an M-rating for a reason. Too-realistic combat that includes disturbing scenes where attack dogs chew out your companion's throats put this war game over the top. Buy the toned-down Nintendo DS version of the game instead, which has a more kid-friendly 'T' (for Teen) rating.
I love the first sentence. If only for this I think this game should stay in the list, but maybe in the 5th place or so.