Halo is teh shit!!! No, just kidding. =P
I have to cast my vote for Duke Nukem. Fuck everything else I've played, this one is funny and awesome.

+1

Origonal Doom - with 3D Glasses ;-)

Damn it, I love all the games that have been mentioned in this thread. So... hard... to... choose...
Probably Doom 2. Yeah. I have spent so many hours playing that.

Mirror's Edge. Although you don't really go around shooting.

how do you do this?

I bet you need to be on acid :-D

I'm going to go with the original Medal of Honor. I flat out LOVED the game when it came out.

Hovertank 3D

Tough choice. For me it would have to be either Half Life 1/2/Ep. 2 or No One Lives Forever. Those games rock. Need rebuy it though as it's scratched so much it doesn't install. Man I wish I could fix that!

There are disc polishers that can fix major scratches. It might be worth looking into if you've got a lot of affected discs already.

Allied Assault was awesome, agreed.

Doom 2 if we are going for pure FPS.

I think I'm gonna say Duke Nukem as well.
Counter Strike Source is my preferred multiplayer FPS