Pretty much any decent MMORPG, as the community can keep you in for years and years. One of my friends with whom I started playing Ultima Online back in 1998, is still seeing his UO mates every week, good 8 years after they parted from playing that game.
MY HYPE LIST: 1) Gran Turismo 5; 2) Civilization V; 3) Starcraft II; 4) The Last Guardian; 5) Metal Gear Solid: Rising