Those were a tough choice to decide on, I finally decided on System Shock 2 as it would not take too long to complete the game. Saying that I have played it a number of times so I can fly through it. Diablo 2 I find needs a lot more time to play as it takes time in levelling up your character and getting through the game. I've not played the new Wolfenstein game but the previews look awesome! Again I haven't played Thief Gold so was not one of my top choices.
System Shock 2, so you can explore the roots of Bioshock. If you've played Bioshock and you're waiting for Bioshock 2, System Shock 2 is a good way to learn some history in the meantime. Make sure you get the high res texture packs.