This one is a bit tougher because I love dirt bikes, but here we go in no particular order.
1. 1991 KX 125 - bought to race in College, but could never get a bottom end bog fixed
2. 1991 DR 350 - weighed too much and was a nightmare to start when dropped
3. 2003 CR 250 - it was one of my flips. I thought about keeping it, but it was very alien in fit and handling
4 1984 KX 500 - Technically my dad's bike, but I rode it a lot. Man that powerband was like a light switch and very evil. It nearly killed 15 year old 5'10" 150 lb Umberto
5. 1985 KTM 250 EXC - another of dad's bikes. Terrible forks and left side kick. I can barely walk with my left foot, much less start a bike.
Whew, that was harder than I thought. Not all the bikes are necessarily terrible, I've just had some good ones over the years. It makes it hard to come up with the bottom five.