I guess I will call out the usual offenders:
Dave Mustaine
Ted Nugent
Lars Ulrich
Geoff Tate
Axl Rose
All of Motley except for Mick Mars
A name I haven't seen in here yet that I really feel deserves mention is Andrew Eldritch of the Sisters of Mercy. Anyone who knows anything about the history of SoM knows he was (and most likely still is) a real asshole.
I'm somewhat on the fence about Blackie Lawless of WASP. He definitely doesn't know when to keep his mouth shut, and he's not too far behind Mustaine in the tinfoil hat conspiracy territory, but I don't thinks he's quite on the level of most mentioned in this thread.
Yngwie's pretty bad, but I don't think he's quite as dis-likable as most mentioned here. Plus I've had way too many laughs at his expense thanks to adam02.
I actually don't think Gene Simmons is anywhere near as bad as most make him out to be. Yeah, his whole Heavy Metal Gordon Gekko image has become rather grating, but he's quite gracious and amenable to KISS fans. And I can't really say I hold anything against him (or Paul for that matter), for ousting Peter and Ace (Ace is a bit of an egomaniac himself and while Peter can be nice, he's also a bit...... nuts)