It's a tossup between Bioshock and Borderlands. Bioshock probably was a more unique experience because of the environment, but Borderlands got so many more play hours, as well as plenty of co-op in there as well. I'm also a sucker for loot.
Oddly, Borderlands 2, though better in every possible way, I just didn't connect with. I loosely accuse my hours in BL1 for making BL2 not as repeatedly enjoyable, but I think it comes down to actually having good characters and a story. Once you go through that once, playing it again seems trivial. I feel some of the most replayable games, like Diablo 2 and Borderlands have their weak stories being a boon to them.
Honorable Mention:
Since we are going to consider PC games of "this generation" being during this console cycle, I feel that I can't finish this post without mentioning Supreme Commander and its expansion. Bioshock and Borderlands made FPS games enjoyable and replayable unlike most games from their genre, but Supcom basically ruined all other RTS games for me. Too good.
Oddly, Borderlands 2, though better in every possible way, I just didn't connect with. I loosely accuse my hours in BL1 for making BL2 not as repeatedly enjoyable, but I think it comes down to actually having good characters and a story. Once you go through that once, playing it again seems trivial. I feel some of the most replayable games, like Diablo 2 and Borderlands have their weak stories being a boon to them.
Honorable Mention:
Since we are going to consider PC games of "this generation" being during this console cycle, I feel that I can't finish this post without mentioning Supreme Commander and its expansion. Bioshock and Borderlands made FPS games enjoyable and replayable unlike most games from their genre, but Supcom basically ruined all other RTS games for me. Too good.