The atmosphere against Real Madrid has got to be one of the best I've seen and heard in years, and that was just through the TV. Horrifically envious of anyone who was actually there, regardless of the result in the end.
As far as games I've attended, the second leg of the Carling Cup semi against City a few years ago was brilliant. Green and Gold (as futile as it ended up being) was in full swing, City just starting to become a moderate threat and Tevez chatting a load of shit all contributed to it in parts but it really was brilliant.
This one wasn't too bad either but I was stuck up in the nosebleeds for it where I was in the Stretford end for the City game:
Gutting that I haven't been able to get to a game since the 8-2.