Which game are you talking about?
Resolved: Online Rating
I found the one I think you might be thinking of: http://www.chess.com/echess/game.html?id=49235183
The thing to note, the best win statistic is taken based on ratings after the game, not the ones the game started with. Your opponent had a 1748 before the game but a 1665 after, which was lower than the rating of your best win.
Not that I have an ego but....I beat a player with a 1718 rating and my stats show highest rated win of 1674. For some reason this bothers me.