The most nerve racking moment for me in a FPS was the Tentacle monster in the original Half Life. Sneaking around that thing and trying not to make any noises was an exhilarating challenge at the time.

What about you?

I made a video of it (1:10 onwards):

Heart was beating very quickly when I knew that the entire enemy team was just around the box xD

Radec in Killzone 2. I got all the way to the end on elite and couldn't for the life of me beat him. Gave up and kept moving. I'm sure I can beat him now tho.

Two experiences in MAG. First was parachuting into an Aquisition match. Bombs going off everywhere, cars flying, people landing on top of buildings, and people being sniped around me. Absolutely amazing!
Also, when going behind enemy lines and blowing up their assets and attacking people who were trying to break through. Oh gosh. I freaking love that game. Just thinking about it gives me a sad feeling inside knowing that no other game exists that gives me those feelings other than it and the player count is already so low. :(

It wasn't difficult like the ones you are all describing, but it was certainly nerve-wracking. Near the end of BioShock, confronting Andrew Ryan, and seeing him command you; being completely unable to control your actions, and finally clubbing him to death despite the fact that you don't want to do it anymore.

Singleplayer: Ravenholm, always. Those fecking spiders creep me out...

Multiplayer: Gears of War (Ranked) myself vs 4. I dominated those suckers though!

I did something similar in Uncharted 2. I had to use gorilla tactics. kind of cheap, but they kept in a group so I had to do what I could. I could have just sniped one and hid, but I have more honor than that.

Well, there was that time in Team Fortress 2 where, as an Engineer, I almost singlehandedly held off the enemy team while my teammates were routed. I lost 4 sentries and killed over 50 guys on one life.

Anything to do with Dead Space and Dead Space 2. *shivers*

Wow i was thinking on that too!
i had nightmares with that monster in the original Half life, i died ther like a thousand of times there...