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
![Tongue](https://bitcointalk.org/Smileys/default/tongue.gif)
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.