System Shock, Bioshock, Half Life, Deus Ex, Thief, or Assassin's Creed?

I've been thinking about getting into some of these series and I was just curious what the general concensus was as to which one is the best. I was also curious about how good people rate them individually, even if they can't compare it to the others, and how story driven they generally are, and more importantly, how good the stories actually are.

Bioshock and System Shock are pretty overrated from a storyline perspective, and Assassin's Creed is "okay" if you're into sandbox style games (though the first one is reeeaaalllyyy repetitive.). Half Life is overrated too. Good for it's time, maybe. But a meh worthy plot and dated gameplay (minus the puzzles!) kind of soils it.

My advice, Thief and Deus Ex are the ones to go for. You really should try them all though.

Of the three that I've played, Assassin's Creed 2 (AC 1 is still solid) is probably the best. An incredible game, near perfect. My only gripe is that it doesn't have the intense chases of the first game.

Bioshock is good, not great. Certainly not the masterpiece people have made it out to be. The main problem is that it works really hard (and succeeds) to have a dark, scary atmosphere, but then you become a super-powered kill-monster and that kinda ruins it.

Thief: Deadly Shadows (the only one I've played) is great imo. A pretty fantastic game and people say that its not even as good as Thief 2.