Alright, a little old school...
I think the most fun I've had with a FPS was DUKE NUK'EM... while playing with another friend.
I remember I had discovered a little glitch with the PIPE BOMB. In the game, you tossed down a pipe bomb, and your free hand appeared on the screen holding a "deadman" button. You could set off the bomb by lifting your finger off the keyboard. The GLITCH was, as long as you kept your finger on the button, you could continue to lay down pipe bombs.
My friend and I were running around this vast map, killing each other, when I came across a long ramp leading up to a drop off into a large room. On one side of the ramp, at the top, was a small alcove where a cache of 10 pipe bombs respawned every few seconds.
My tiny brain came up with a giant plan.
I hid in the alcove, and started filling the room below with pipe bombs, laying more down as they respawned. The PLAN was to wait for him to come up the ramp, let him see me at the top. I would jump down into the room, leave out the exit, and he would hopefully chase me, jumping down into room full of pipe bombs.
I had over a hundred pipe bombs in the room, when I decided to peek down the ramp. He was halfway up! He started firing, I panicked, and jumped down into the room... but my fingers slipped off the keys.
The room went up, and my many body parts decorated the walls of the room. There was a short pause, then I got a message over the screen from my friend, "Uh.. you okay there, buddy?"