This is an old one, (Actually two) but it's friggin legendary in my mind

I was playing Rainbow Six: Rogue Spear deathmatch many, many moons ago when my team and I were stuck outside a hospital, unable to get inside as one of the other guys was covering the only entrance with a sniper rifle & there were only a couple of columns and a truck to hide behind, all of which were in his firing arc, so we were getting popped left and right without even being able to fire a shot.
This was pre-team speak so after about 2 minutes of typing & I came up with a plan. I got two of my guys to distract the sniper while I ran straight to the door and ducked down. I waited until I was absolutely sure he was zoomed in before I made my move, and actually snuck right past the sniper's scope and around the corner so he couldn't see me. He was set up behind the "nurses station" which only had one way in or out and had perfect cover. It was pretty much impossible to get a shot off or even lob a grenade without getting shot by the sniper or one of the other team members who were hiding in the map. It was pretty much a camper's wet dream.
Anyway, I stayed ducked down and crept right up beside him, pulled out my pistol, aimed it at his head, and in an ultimate act of smartass-itude, actually risked taking the time to type out "BANG" before I put two in the back of his head.
The chat was then filled with "Holy shits" and "That was fucking AWESOMEs" from people who were ghosting (even on the other team) and of course, one douchebag accusing me of cheating.
Same game, same map, I had figured out exactly where to aim and worked out the timing to be able to get a pistol headshot every time, which of course caused more cries of "CHEATER" when the truth was I just played that fucking game waaay too much.
In the 10 or so years since, I've done some pretty sick moves in multiplayer games but nothing so far that tops that sniper ***** slap
EDIT: Okay there WAS this one time during a BF2 match that I found a perfect sniping spot overlooking an airfield where I must have popped at least 6 guys as they were running for aircraft, but the best part was the fact that I was so far away that even in my sniper scope I could barely see them, but I led them all perfectly & even scored a single headshot before a helicopter finally spotted me and fired about 10 rockets up my ass...