L4D2 is the bizzle.
Get L4D1 too if you haven't already, it's also the bizzle.
Wolfenstein (AKA RTCW2) is cool, straight up old-skool-ish shooter with some cool extras that actually make it more fun.
Bioshock /
Bioshock2 for something with an RPG-lite flavour and a different setting.
Fallout3 for similar but more RPGish.
Singularity is meant to be pretty good fun and sounds and looks it too.
Presumably you've played all the older stuff like
D3 Q4 HL2 FC1 COD4 etc etc.
Assassin's Creed isn't a FPS of course.
Hurrah for posting about proper games on a proper system rather than all this console bollox
I'm going to upgrade in autumn for the dank winter nights...