You asked which games are longest. I haven't played many of the games you listed, but STALKER:Shadow of Chernobyl gives you your money's worth even if it isn't as long as a Bethesda game (but how many games are?).
I enjoyed the game "Prey" including
Spoiler some memorable and intense scenes with children's ghosts
. It has an interesting game mode called Spirit Walk where you have to send your spirit out of your body in order to scout some areas, open doors, or even fight. It's not a replacement for the FPS parts of the game, it complements it. The game is relatively short but has a memorable introduction scene and Art Bell on the radio.
Hellgate:London single-player also lasts a while because you're having to travel between former subway stations in postapocalyptic London, alternating with street fighting and Hell dimensions. I recommend playing a summoner (who has elemental demons) or an engineer (flying drones), though the sniper is interesting too and there are two blade-users if you want to go the melee route. Don't expect it to be one of the best games you've played, but you might enjoy it.
I finished Bioshock 2 recently and it was good, though not as fresh as the first somehow. But it has its moments, especially
Spoiler the level when you see Rapture through the eyes of a Little Sister: they see a beautiful version of Rapture and splicers instead of the reality. The level has to be seen to be believed; I understood immediately what the devs were portraying.