Fighters, BY FAR. I have never understood the appeal - one friend of mine enjoys them because there is strategy involved; it takes time to learn the moves of each character, and find out which moves work best against which character, etc. However, not all fighters are made in a balanced way, and there is more often than not one or two characters who are so overpowered that they wipe the floor with every other character.
Still, that's not my biggest complaint - the entire game is played on a 2D platform with some back and forth punching in rounds that last no longer than 90 seconds. Stories, if they ever exist, are weak and ignorable. These games just seem so flat and transparent (appropriate, considering what one sees when they play the game) and have no intrinsic value as games whatsoever. In other words, they FAIL.... really, REALLY hard.