I don't know much about Kavanagh's work primarily because I haven't read a lot of Web of Spider-Man since I've always found that title to be lackluster, mostly inconsequential and not worth reading. That being said, he was probably the worst writer during Maximum Carnage and The Clone Saga, so I can understand why a lot of people choose him as the worst.
Mackie does have some good stories before the relaunch, but O'Neil's Spider-Man work was thoroughly boring and highly forgettable to me. And I'm sorry to say that I'm also one of the people who hasn't liked Zdarsky's Spider-Man issues at all so far. I respect your opinion though.
And once you have heard of him, you can never forget him. Ever. Believe me, I've tried.
Look, maybe i am being a bit harsh. Obviously there were a few issues penned by Kavanagh (towards the end of his run on Web). However they are few and far between. I also think Alex Saviuk's pencils were the reason i ended up enjoying those issue more than i normally would have.