Aliens vs Predator multiplayer beats Fallout 3 for most immersive experience IMO. You genuinely get quite nervous or even scared when doing AVP multiplayer, or at least I do.
<spoiler=One of my fave AVP multiplayer games>I was going along a well lit tunnel, and halfway there I hear a faint *crash* as a light is smashed behind me. I suddenly turn around to catch the Alien off-gaurd, but I can't see it because the bloody light's gone out. I start frantically scanning the area with my torch, and find nothing. Just then my motion sensor starts going crazy, as something moving passes over my head. I look up, then around again. Still nothing.
The beeping of the sensor stops.
I calm for a moment, before a light right in front of me smashes, catching me off guard. Quickly the other lights in front of me smash, and I run into the nearest room, which has a few light in it. Faint ones, but still lights. I cower under it, knowing this will show where I am but I know the Alien can see me anyway, and I can't see it. Frantically hovering my torch all over the room, I honestly jumped in my chair as the light JUST above me smashes. This did it for me, I started spraying my bullets all over the place, missing every one.
Reloading, and I lose control of the Marine, knowing I finally died. The Alien got me.
So I respawn and await something similiar happening.....
[/spoiler]
Sorry, thought it best to describe my experience. It was just SO tense and incredible...