Final Fantasy.
Spent over 20,000 hours playing these games on the PlayStation, and I've written countless Walkthroughs and guides on them. I've completed all the games 100% at least five times for each game (there have been 6 for the PS so far), and many times just to get some storyline/boss specs right. Safe to say that if my grandma hadn't bought me FF IX when she randomly stumbled upon it in a shop, my life would have been *very* different today =). My favourite FF game is Final Fantasy VII, the greatest game ever made.
I've played many other games though, Gran Turismo is still a favourite, completed Gran Turismo IV 100% two times now. Played RuneScape for a couple of years too, earning me a rank 100 spot, but I quit it to start Warhammer Online when it comes out in a month or two. Looking forward to it a lot like the childish being that I am!