Loved GTA: Vice City since it was the first game I'd ever played where you could kill people in a realistic way. Appeals to any 12-year old, I'd say.
San Andreas was even better, apart from the story, and I could spend hours just cruising around on my motorcycles.
Didn't get very far with GTA IV however. The handling on the vehicles were shit and the constant cellphone calls drove me nuts. Ment to try again later when I quit playing, but I don't see it ever happening considering it must've been over two years since I got it.
Doubt I'll pick up GTA V, if it's ever being made, unless they've fixed the flaws from GTA IV.