A Najdorf Sicilian which brought home to me how careful you have to be in these highly theoretical lines. My opponent goes for an aggressive king-side pawn storm, but completely underestimates the strength of the queenside counterplay I have available. Once it becomes apparent, around move 15, it's all one way, and white never recovers.
The Najdorf is one of the lines I'm exploring at the moment by sending out chess.com challenges starting from a specified position. I'm finding it very helpful to help me learn theory. I'm sure in the next few weeks I'll have an instructive example of how to lose as black in the Najdorf, but until then, I'll stick to posting wins!