For me, it's easy, the 3-0 away win in our memorable League Cup run in the 70's.
They were top of the top tier, they put a full team out and we crushed them. Their fans couldn't believe it as they had beaten Man Utd 5-0 on the Saturday, and then up we come, 3rd tier and humiliate them in their own back yard.
Walshy outshone Stan Bowles that day.
Yours...................? (No fannying around, you can't have two, it has to be THE one )