Thought this be a fun overview of terrible kits.
The very worst model kit I ever encountered was hands down Alan’s 1/35 SU-76M. The plastic was poor quality, extremely thick and nearby devoid of detail. Fit was pretty horrible as well.
Honorable Mention:
MiniCraft/Academy for their 1/48 F14 Tomcat which was the worst fitting aircraft model I’ve encountered.
Lindbergh 1/35 T55 lack of detail and poor fit.
ESCI for 1/35 T55 & Leo 2A4 due to major lack of detail
Definitely for me the 1/72 Airfix Tiger 1 and M4 Sherman. (the old kits)
The Skif 1/35 MTLB…one of their first kits. 
at the risk of offending someone, the Miniart base I’m using for a campaign. having never used Miniart, I was CONFUSED!
Cheers!
L
Without doubt Military Wheels 1/35 “UAZ-469 with 106mm gun of Northern Alliance” 
/Freddy
Trumpeter T 55, oversized sprockets, tracks to wide.
Second place for Trumpeter 1/16 T 34 mod. 1941 mostly due to ungluable styrene which was used.
It was wery difficult to get proper bond between joints.
I haven’t run into any “ horrible” kits where fit was terrible. And such. Most difficulty is on my end.
I did build the Tamiya Hanomag 251 a few years ago. It was truly boring. I did add etch and scratch rear seats.
Anything that has individual link tracks! The very definition of purgatory. I take my hat off to anyone who can work with them.
Honestly though, the worst armour kind t that sticks in my mind was Mirages 1/35 Polish TKS tankette. At the time the only game in town. The hull, while a little toy like, fit pretty good. The problem was the suspension units. Bad fit and barely any mating points to fit to the hull. I think it lasted a night before it fell apart.
I actually recall the ancient Airfix stuff with considerable fondness; each time I built them, and the Tiger and Sherman were often repeat buys while I tinkered with my own version of wargaming, I seemed to improve so perhaps all part of the learning curve as I was growing up.
Where I despair is with the Bronco GAZ 69 and the Roden M37. Both are, to me essential vehicles for a Cold War collection but I’ve just about given up on both. Too many parts - most of which seem to be unnecessary - and poor fit, to such an extent I have no confidence that the final stages will correspond at all. If anyone has tackled these successfully let me know - I could use the encouragement!
Horrible…soft details and horrible tracks. Still got it done…after quite a bit of alcohol and foul language.
For me it was a very very old Panzer 4 H from Academy. What an absolute hideous contraption… A complete dog on a biblical scale.
Nothing fitted or lined up. There were sink marks the size of the grand canyon. The frame for the turret and hull plate armour would not line up or fit to anything to make it symmetrical.
I spent about 6 hours sanding, filling, trying to line bits up then just couldn’t do it anymore. Into the bin it went. Every single bit of it. Only kit I’ve ever binned.
Oh yes, I built an Academy Pz IV and it was riddled with sinkholes & sink marks exactly like you said. Total box of garbage.
Thank god it just wasn’t me…it actually put me off Panzer 4s.
Yes, the Panda MATV in 1/35. While the basic geometry and detailing is pretty good, the fit, particularly in (but not limited to) the suspension is not good, requiring a fair amount of cleanup to work, lots of cutting, filing, and dry fitting. Of course, I’ve now built several of them! haha - oh well - maybe I’m now a better model builder because of the experience.
Nick
Airfix 1/76 Tiger I. Horrible kit and the rubber tracks were short and bent the idler wheels.
Bodeen; ditto for me, I also struggled with that one, the Alan kits of a certain age are truly horrible. Their only redeeming feature was that at the time they were the only game in town. I can’t think of any good reason for building one now. You’ll be pleased to know however, that having read other reviews the Gepard actually wasn’t their worst!
That’s the first I’ve heard about the unglueable plastic
For me it is another Alan kit: The Bison I based on the Panzer I chassis. Box was missing half of the instruction pamphlet (a nice person sent me a scan) and it asked you to put in pieces in an order that required breaking/removing parts from previous steps to get them in. I now have the Dragon version to replace it.
Worst I can remember is the Dragon 1:35 M4A4 - Almost every part needed major surgery to fit! And the clamp-use was pretty fierce too…
I have never bought an Alan kit due to all the horror stories of their kits.