Not got the memory to rank them properly. But here's some shockers off the top of my head
Lost 2-0 at St Andrews in 2002. Andy Smith came on at one point. I'm not sure we had a shot.
The frankly horrific 2-0 defeat at Oakwell, March 97. Barnsley coppers celebrating with pitch invading Barnsley fans while United make no attempt to even compete
6-0 at Man City. Tracey sent off. Horlock, Goater and Gareth Taylor scoring, I don't mind. Dickov and Mark Kennedy

The surrender at Villa Park in the penultimate game of 06/07. We didn't even try. Anyone wanting Warnock back should remember this day
1-3 v Notts County, August 1994. I was only 13 but I remember thinking "my God, the horror". There were only 15k there as well. Littlejohn scored for United (I think)
4-0 defeat at Wigan, September 2004. Nathan Ellington annihilated Jags