Why They Don't Deserve It
The seething rage that seems to accompany any discussion about Fall Out Boy is in no way related to their music; anyone who claims otherwise is lying to you or themselves. Understand, I'm not saying FOB is the greatest band of their generation, but musically, they aren't that bad. Patrick Stump can write a decent song, he's got a great voice, that Pete Wentz dipshit jumps around the stage all acrobatic-like, which is kind of fun to watch ... it all adds up to something in between "good" and "at least not as terrible as you claim." What they definitely are not is the worst band of their or any other decade.
Your title is safe, Nickelback.
So what made people so passionate in their hatred of this particular band above so many others? I think it was a few things. For starters, once you've reached a certain level of popularity, some segment of the population is going to turn on you. Whatever rock band happens to be dominating the radio at that moment is going to be singled out as the reason why current music sucks. For a few years there, Fall Out Boy was that band for one massively easy to mock demographic -- teens. More specifically, "emo" teens.