GTAIV Online: based on how hard I've laughed for the most amount of time.
TF2: based on how long I've played.
Civilization V: based on personal preference (hooray for
TiFe.)
Also, Minecraft is fun with friends, if you don't have any other creative outlet (I do so I don't play much anymore.) Starbound isn't bad, either. And Rougelike platformers are great, I believe this includes Rogue Legacy, Spelunky and the recently released Katacomb Kids which I am looking forward to. Oh--the Demon/Dark Souls franchise is also fun, if you're into sadomasochism. Not to mention a few classics: FF7, Super Mario series, Poke-a-mans (the old ones that didn't involve
ice-cream shaped monsters), Marvel vs. Capcom 2, Castlevania series, Metroid series...
Portal & Portal 2 are a couple more greats, albeit short. The Last of Us is a good choice if you love stories. Smash Bros. Brawl is hilarious with friends, though that happens rarely nowadays what with online gaming
Psychonauts is dated but it has one of the strongest personalities in gaming to date. So does Hotline Miami; lots of character and style. There's also FEZ...
...and Bastion. And Amnesia: TDD. And Cave Story.