I can give credit to Quesada for turning Marvel around after the crap comics of the late 90's by flooding the market with Marvel products, TPBs, and he did help create MAX and Ultimates. I also like that he tried to stop the whole 'die, return from the dead' crap. But as a writer, he sucks and his Brand New Day SM was ridiculous and probably the worst retcon in Marvel history. The guys at the LCS rag on him alot.
Bendis is a much more interesting writer. His writing on Avengers, Daredevil, and House of M are all great reads. I think he also was the primary writer for Ultimates, but not 100% sure. Secret Invasion sucked, but overall I would credit his writing as adding to Marvel's success.
Quesada takes the prize as most hated...