This was bad, but the 1-4 at OT vs Liverpool in 09 still remains my most hurtful memory of watching United. We were under Fergie's best possible spell as manager, going into the stride to win the title, the team was filled with quality players and the game felt as the one we would decide the title race in given Liverpool was the other contender.
We took the lead from the penalty - but then that Torres goal leaving Vidic flat on his ass who was probably the best defender in the world and something that still gets brought up when he is discussed, and after that it just kept going bad to worse. Gerrard scoring and kissing the camera, then Vidic sent off and we concede from the resultant free kick and to end it all the absolute kick in the nuts with fecking Dossena scoring a chip from distance over Van Der Sar. It's one thing having Mo Salah scoring, quite another having an absolute no mark player embarrassing one of the best keepers we had who had set league records that year.
I think for Liverpool fans as well that would probably be a better memory than today. Especially as it gave them title hopes, and given they got it with Fergie in charge and dishing out a humiliation at OT at that time would have been a lot more satisfying.