Guitar Hero.
If I want to tap my finger along with a tune, I don't want a ****ing computer telling me I'm doing it wrong!
Sure, there are loads of games that are less well executed and that others may think much worse than Guitar Hero, but at least you got to kill something, blow something up, crash into something, make some blocks disappear, walk into walls, jump into a spiked pit or just type in swear words.
Guitar Hero appears to me to be the video game equivalent of standing in front of a mirror with a tennis racket and pretending you're Jimi/Slash/whoever, but not nearly as much fun. Pretending to play a guitar with a tennis racket isn't considered cool (I think), why is pretending to play a guitar with a big, stupid, overpriced game controller so popular?
Also, I'm not very good at it, but I don't care, because I think it's a stupid game and I don't like it!