DOTA Game. I was still very new and didn't understand what items to get and how to get them, etc. We were losing pretty badly and I was playing sniper, the whole team left except for ONE guy who hated me, and it was us vs the whole team. The teammate who hated me, coached me, and we both ran an amazing defensive game and frustrated the other team of five players so that they left one by one until the host finally left and we all dropped. World Tree defended.
TF2. We were playing one of the bombcart levels, I forget the name honestly. I was on Demoman, Blueteam, offense. We ripped through the redteam in short order, pushing to their final cap quickly, but then someone on their team left and I died, so I got AUTO balanced to the RED Team. I don't know if I'd say it was ME that turned it around, but I proceeded to rack up over a dozen kills and only three deaths bombed the hell out of everyone who tried to push the cart into the final area, as well as lobbing grenades through windows and onto catwalks. In the end we held them off until time ran out and Red Team won the round.