Worst mtn bike was a '95 [?] Stumpjumper FS Comp. The bike itself wasn't bad, but the Bike Shop guy sold me a 19" frame. My last 4 bikes have been 17", just as a comparison. This was the beginning of my distrust of The Bike Shop. I should have known better, but it sorta fit in the store and on the 2 minute test drive...
Worst quality road bike was a Woolco 10-speed I got when I was 12, but it carried me everywhere I wanted to go, so it wasn't that bad. The bike that really was the worst to ride was an IRO Rob Roy frame/fork which I built up into a daily commuter. Holy crap, what an awful ride! It didn't help that it was a boat anchor as well. "Double-butted" my ass! Gaspipe FTL!