My greatest win in a game was with the black pieces against Varuzhan Akobian. See Chapter 3 of this study:
lichess.org/study/kVinOKTDMy second greatest accomplishment in 1 game was that I beat a 2550 GM with the black pieces. His name was Victor Mikhalevski.
This is the game:
http://www.viewchess.com/cbreader/2016/7/1/Game30105150.html