Fallout 3 if you want a good, long, single player RPG/FPS.
Bioshock 2 is very fun, pretty much just like the first one. The single player won't keep you going for too long though. Dunno about the multiplayer, haven't tried it yet.
Borderlands is very fun if you have 3 other people to play it with, but it's just a mindless lootfest.
Fallout 3 is by far the most "RPG" of all of them. I put 65 hours into it with 3/5 of the DLC in two playthroughs (DA:O took me 55 with all of the DLC on one playthrough, one of the most fun games I've played but I don't think I could take a second playthrough at that length). Although it can be pretty short if you skip the extra stuff. But I'd recommend it for sure.
p.s. I didn't play the first two Fallouts either...