OMG, Brad, you don't like the original GI Toe?!? That's one of my favorites! A giant toe wrestling an ape to the ground (on the side panel) is so bizarre! Interesting too that I kinda like Botch now because it has so much black on it - an odd variation to me. Funny.
Anyway, I went through the original series I knew as a kid (2nd through 14th) and notice that the ones I didn't like much back then are still the ones I don't like much. They tend to have "flat" characters on them that -- like Nooseweek (which is definitely very high on my list too) -- just don't fit, or they don't have any characters on them at all. The same goes for the 1st, 15th, and 16th. Definitely a pattern for me.
I know you asked for one, but I had to list more than one since the first one that came to mind was "Nooseweek" - lol!
1st - Spray-Nit, Cover Ghoul - Although not flat drawings, the characters on these freak me out. Really dislike them. Spray-Nit especially.
2nd - Blecch - Dull gag and no characters. Bottle also somewhat "flat" (e.g. "Skip" from the 9th series is ok because the artwork is good)
6th - Valveater - For some reason the guy on this one REALLY bugs me! No idea why! lol
7th - Medi-Quak - The duck is very "flat" imo. Artwork doesn't feel like Wacky artwork; feels like a non-Topps company
7th - Caged - Flat artwork on dog. And man I really want to make that "g" look like a "g" and not a "q" !!!
13th - Nooseweek - Flat artwork that doesn't fit (as others have described)
13th - National Spittoon - Same as Nooseweek
15th - Ultra Sheep, Sore Mel, Moron - Gags are fine but again, artwork is flat and feels like a non-Topps sticker
16th - Fling 'ems - Flat character; gag is wordy and yet unfunny at the same time