My roommate and I were playing Doom II deathmatch, I was just about to enter a teleporter when he hits me in the back with the supershot gun - killing me outright - before leaping through the gibs to enter the teleporter himself. What he wasn't counting on was my corpse having sufficient momentum to follow a split second later - he was telefragged by my remains.
I doubt I'll ever have a multiplayer experience as awesome as that

Another memorable match involved a chaingun exclusive map I made. This sounded cool but we were too evenly matched and we basically took turns killing the other. At one point I edged in front by killing him twice in a row but it left me at 1% health. And then I pulled off a flawless kill, it was pretty cool.

Boom and ZDoom were lots of fun, I had a lot of fun making levels in ZDoom.