A lot have already been mentioned, but my personal ones -
Chelsea 8-0.
Bradford over 2 legs (I actually bought return flights for the final, and still get reminded of that by mates at work).
Watford at home- I knew Deeney would score and resigned myself to relegation after that defeat.
The shafting of Remi Garde in the transfer window, although I had resigned myself to relegation to see it spelled out by the club was hard to take.
Liverpool 6-0 defeat- I found this the hardest to take, it really felt personal, like someone was kicking the life out of a family member.
The FA Cup final defeat to Arsenal I found OK, we were easily beaten by a better team, I was there sat close to the Arsenal fans, and watching them celebrate just made we think that one day our day will come, and Villa fans will celebrate an FA Cup like nobody has ever celebrated it before.