Update:
Mw3 - Killing 6 people with an XM25 on Hardcore search & destroy.
Bf3 - Running over 5 people in under 4 seconds.
Disgaea 4 - Beating Pringer X
Soul Caliber V - Defeating someone online when my bar was extremely low and my opponents bar was still on 3/4
DCUO - Beating 4 heroes by myself.
Been playing games so long that there is no "greatest" just "Recent"
Was playing Castle Miner Z and some asshole kid joined my game and started mining the ROCK I was standing on... I then fell all the way down to hell but somehow managed to live with .5 health
Finishing Resident Evil 2 and 3 without saving and walkthroughs or cheats, and yeah whenever I die back to square one. ( my memory card broke during those times and my parents won't buy me one T_T)
I also got to disc 3 in final fantasy Viii where you fight that weird looking woman but got my ass beaten and yeah I stopped and accepted it was too tiresome that I really need a save file. So I made myself and parents a deal to buy me a memory card in return for doing my academics better.
I was playing Rush on Bad Company 2, I was on the defending side on Arica Harbor, at the first base, I was operating a tank and just randomly fired at the attackers spawn, hit a Humvee with a full 4 man squad riding it.
Playing Star Wars Battle Front against a friends we got down to just one man left each and we had a sniper stand off that fight alone was more than half the battle. I won at the end but it was a really intense fight.
Tribes Ascend: getting a multi kill my first time playing(I threw a grenade near the enemy flag and four infiltrators died)
Mass Effect 3: beating the game in 3 days(2 days if you don't count the times I slept)
Katawa Shoujo: beating the game in 2 days