Pretty much anything pre 1990 that had the Hobbycraft label on it. They cleaned up their act around then (I think that's when they hooked up with Academy). Prior to that they pirated a lot of other folk's kits and generally churned out total crap.
Notable pre 1990 Hobbycraft kits I attempted but gave up on:
1/72 S-2F Tracker:
Bad copy of the Hasegawa kit with serious mold mismatches on the landing gear legs.
1/72 FW-190A:
No idea where they got the tooling for it, but it was total garbage in every concievable way.
1/72 Sukhoi SU-17 Fitter:
A total joke of a kit and excedingly innacurate representation of the subject even considering that it was made while the Iron Curtain was still up.
The worst kit that I ever saw through to completion would have to have been the old Revell 1/48 F-16A kit from the early 80s. Horrible fit issues everywhere. A Number of years later I bought Revell's 1/48 F-16 ADF kit only to find the old F-16A molds along with a new sprue of parts to make it look like an ADF. It went straight to spares when I saw that.