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.
I see so many great titles in your post!
GTAIV Online - TeamF.2 - Civilization V - games for every player around the globe!:)
I never played Minecraft (haha hard do believe? but I did not )
Portal & Portal 2 - game idea 100/10 - game 10/10
and about the rest I will say something later on !
Thanks!
Cheers!