This is going to be shorter than my favorites list, partly because I don't have as much to say about writers whose work I didn't like as I do writers I liked. In reverse order:
5. Marv Wolfman - Not really bad writing, given that most of the melodrama and such were elements of the period, not Wolfman in particular, but some of the weakest Spidey because it felt like Wolfman never got Spidey. His Peter Parker was the biggest sad sack and failure on the planet, his supporting cast felt more "larger than life" and DC-like, his original villains were pretty terrible and his use of the classic villains not particularly inspired. Still, compared to the others on this list, it was mostly flat writing, not bad writing.
4. Terry Kavanaugh - I confess, I'm not even sure I spelled his name right. But for a while, Kavanaugh was a Marvel editor turned writer in the '90s who contributed a lot of the clone saga and other execrable Spidey stories. To the best of my knowledge, he never wrote a single story I found palatable.
3. Howard Mackie - Which, technically, should put him higher (lower?) on the list than Mackie, who did write a few stories I found at least solid, if not inspired. But Mackie contributed not only to the clone saga, but also to Byrne's Chapter One era, and was involved in so many terrible stories that he's more memorable (and not in a good way) than Kavanaugh. I'm trying not to extend extra credit for how badly he screwed up the X-Men in the '90s and 2000s as well.
2. John Byrne - Chapter One was everything Ultimate Spider-Man could have been. I do not mean that as a compliment. An attempt to "update" Spidey's origin succeeded only in making him dated for the '80s, not the '60s (and this was written in the late '90s), the attempt to tie up "loose ends" of his origin by tying in Doc Ock or having the burglar notice Peter buying a computer totally missed the point of the character and had all the feel of Byrne's usual "continuity fixes," which fix problems that only a crazy person actually saw before they were fixed. The best thing I can say is that this era is utterly forgotten, so it didn't do any long-term damage.
1. Joe Quesada - I know, I'm not even sure he's credited as co-writer, but let's be honest, the blame for Spidey's devil divorce lays squarely on Quesada's doorstep. Straczynski wrote it, and deserves part of the blame, but Quesada was the guy beating the drum for it, and swearing up and down that they had a clever way to un-marry Spidey. An actual divorce would have done less damage to the character. Mary Jane being murdered by a villain would have done less damage to the character. And the "Brand New Day" status quo shackled some very good writers so that they weren't able to get their feet under them fully for months or even a year afterward, and still causes problems. Dan Slott should be a dream writer for Spidey, and he would be if he weren't saddled with the "Brand New Day" status quo.
Honorable mention here to Kaare Andrews, whose alternate reality tale Spider-Man Reign is the bleakest sort of fan-fic crap that Marvel actually published. Mary Jane dies of cancer because of Spidey's radioactive jizz. Seriously.