mario kart was good, but for replayability, graphics, sense of community, storyline and overall cool factor iam going to have to go with halflife 2.
Ive played through the single player a fair few times, and all the multiplayer mods like cs and dod more than make up that side of things. The story is amazing, all the cool things like the hovercraft boat thing, the buggy, the soldier antlions, it all comes together into a fantastic game.
Hours played is WoW, they win community hands down
Dota comes in there somewhere aswell, team work, pure skill and a shitload of hero choices and skill combinations make for a great game, shit community though, full of laggers, leavers, hackers, rage quiters, plug pullers, general childish assholes, its rare to even play a full 5v5 game through to the end unless you pre organize the game with friends. Basically those people ruin it for me, but the cheer joy i experience from a good fight, using all my skills, teamwork and everything then getting that kill is fantastic.