Razorback was pretty lame.
Spider-kid see below
White (or red) Dragon from PPSS
Enforcers became sidekick villains 4 issues after their first appearance and I'll throw in Princess Python as well.
Meteor Man was another Ditko miss.
Mind Worm
The blob from when Hydro man and Sand man get mixed up. The worst part about it was a complete King Kong ripoff and then at the end of the story they have the gall where Spiderman says something to the affect of "Holy Cow this is just like King Kong - wowee"
Madame Web pretty much sucked.
The Slide was a rocket racer ripoff, but I always loved the cover of his first appearance - very Ditkoesque. Worst... origin... ever. A chemical inventor has a spill while messing around with Teflon.
My vote is for Spider-kid, I remember cringing when that issue showed up in my mailbox right after what I consider the greatest run in ASM history 238-261. Hobgoblin after Hobgoblin storyline, Black Costume storyline with MJ origin. Even the filler issues were good with "the Kid who loved Spiderman" tear-jerker. All of a sudden we get a crappy photo cover and secret ID story and then this piece of