The worthiest of games...

Just out of pure curiosity, which game do you (excluding the below), consider the best one - using a criteria below stated. I'm thinking about getting a game, and the price of games here is ridiculous, so I really need to think this out.
I have: Gears of War 2, Halo 3, Far Cry 2, Rainbow Six Vegas 2, Quake Wars: ET, PGR 4, Fable 2, Oblivion IV (w/ Exp. Packs), Devil May Cry 4, Ninja Gaiden II, Infinite Undiscovery, Eternal Sonata, Bullet Witch.I'd like something that would last me a while, either with a good SP campaign/extras or perhaps with multiplayer/versus.Just a tip of games I've been considering, lately: Borderlands, Modern Warfare 2, GTA IV, Resident Evil 5, Assassin's Creed, Assassin's Creed II, Bioshock...They don't need to fit into that portfolio, anything that you consider good and have reasons to justify why it is good is enough.

The worthiest of games...
Dragon Age: Origins, Bioshock or Borderlands I'd say are top choices. Borderlands multiplayer is good enough it'll keep you in for a while. The other two games are pretty long games. Bioshock even has 2 endings so you'll be doing 2 playthroughs just to see them.The worthiest of games...
If you have a friend that has Borderlands then I would choose that. Either that or MW2 because Borderlands has a pretty long story and good replay value and the multiplayer for MW2 is good even though the campaign is short.
Assassin's Creed II, but it's best if you play the first before the second. It's not exactly short, but it isn't incredibly long either. But, it is insanely fun and the storyline is fantastic.
I put a good 50 hours into one playthrough of Borderlands.