These are the games that kept me playing for hours and hours.
World of warcraft: I've played a handful of MMOs but nothing is compared to WoW with its simplicity. There is always something to do in Azeroth. Anyone can pick up a copy and start levelling a character and easily form friends and join guilds. Just be ready to sink lots of hours with this game.
Fable II: Game of the year 2008 to me. Simple controls, exciting story, moral system and DLC. Can't ask for more :)
Teenage mutant ninja turtles: One of the few old school games that I still play. Super fun played co-op. Michael angelo FTW!
Gran Turismo 3: The game that defined racing genre. Sinked hours and hours customising cars and trying to finish all trophies and getting gold in all tests.
Capcom vs SNK 2: Also one of the few old school games that I still play. Largest selection of fighters that I know of, different grooves to suit your need and favourite capcom and snk fighters in one game.