Pokemon. I've played my Y version for over 500 hours alone, and all of the other Pokemon games have had hundreds of hours logged as well.

Been spending Hours and Hours on Resident Evil Revelations (Wii U) everyday! I know I put 40+ hours playing RAID mode... just addicted

The Age of Empires series, especially II. I'm addicted to it all over again on Steam. Spent a good few hours on WoW, and I've put a lot of hours into COD BLops. Xenoblade has taken over 100 hours of my life.

I have spent around 100hours plus on toy story 2 for the PlayStation. Though I usually spend 10 hours to 15 hours on most platformers I play.

Urg.. my mmo days... Everquest 2. I was obsessed for a year and haven't touched an mmo since.

Actually, I credit that game with giving me a low tolerance of any game the involves grinding levels or mediocre sub quests. It's a big reason why I really couldn't get into skywind despite being a fan of oblivion, morrowwind and redguard back in the past.

ridge racer V (spent about 10+ years and still counting)

honorable mentions : metroid prime,super mario galaxy, ocarina of time,baten kaitos, fzero gx, sm64, resident evil 4..

About PS3/360 gen:

Fallout 3, PS3 and PC (PS3 good karma, PC bad karma), 500 hours in total
InFamous 2 because I played it a lot of times, Little Big Planet and Modnation Racers for the multiplayer and PlayStation All-Stars Battle Royale in local multiplayer with friends (almost every day since release xD)

About the PS2 gen: Jak 2 and 3 because when I was teenager I loved messing around with dark powers in Haven City

Counter Strike Source, but now the game I spend the most time playing is Titan Fall.

League of Legends: Being that I have probably 2000 games, I'd say it is somewhere between 1000-1500 hours.

Kinda sad really, being that it is not even remotely close to my favorite game.

Just remembered WoW, maybe 1600 hours? Mostly in the 2-3 years after it came out I don't like what it has become nowdays