I'll get the ball rolling...
Randy Rhoads was not that good. Yes he was a great player, but it's cartoon heavy metal, it sounds silly & tacky. Only the bad taste bastards of the metal world would think that neo classical rock is a good idea (just the term makes me wince).
Bob Dylan... so fucking what? Don't get it. Some stuff I don't get but I see the appeal. Not Bob.
The Clash were just middle class chancers making pop rock and are only so big because most people don't want to be challenged, but like the idea that they are. I say this as a fan of the clash! But the idea they were punk is bullshit... they were a pub rock band who pretended to be angry interested in politics and social issues because that's what constituted the latest fad at the time
A friend of mine once said that if Corrosion of Conformity had released "Load" instead of Metallica, it would be hailed as a classic - spot on. Not a massive fan of Load but it gets unfairly slagged
The Beatles = overrated. And before anyone (not that I have you lot pegged as bastards, but slagging the beatles usually gets a bad response) gets all pompous, tells me I am a bad person or stupid for not liking them and lectures me on how important they are to the development of pop music (really?! no shit. wow you must have watched television or read a paper at least once or twice to gain such an insight, I am in awe), I know - and I do like them. But for me the myth is as powerful as the music - that's why, when you say they are overrated, people always refer to the band's place in history rather than the music itself. they didn't invent beat pop, they didn't invent psychedelia, they didn't invent rock music or heavy metal... let it be!!!
Mastodon's drummer is so good he's shit - ditto Steve Vai. But only virginal bedroom guitar heroes stick up for Vai so maybe that's not so controversial