Liam Gallagher would have to make my list, but he is bad in a funny way at times. He claims to be the reincarnation of John Lennon, despite being born in 1972.
Noel Gallagher, on the other hand, comes off as a smart, funny, decent guy in my opinion. I've seen him in interviews, he's really only bad to his fans if they are pushy.
Gene Simmons is an obvious one.
Lindsey Buckingham is an arrogant, controlling asshole from what I hear.
Courtney Love is a trainwreck who shit-talks people for a living.
Axl Rose is an aggressive, over-reactive prima donna.
As much as I love Pink Floyd, Roger Waters is a dick.
On the other hand...
I've heard good things about Richard Ashcroft from people who have met him.
Chris Martin seems to be a really open and down to earth person who writes about real things and stays true to his style.
Thom Yorke, while being shy, once stopped a concert when he saw someone in the audience having a seizure and made sure he got medical attention.
Colin Greenwood seems to be a fairly nice person, he's often referred to as the peacemaker of Radiohead.
All of Rush seems to be really nice.
Every member of the Dave Matthews Band also seems to be pretty nice as well.
Hank Williams, Jr. is an EXTREMELY nice person, and I'm convinced of that by a story I've heard. Basically, a guy had a heart attack at one of Hank's concerts, so he stopped the concert and made sure he got medical attention ASAP. Hank then visited the guy in the hospital and they kept in touch.
Dave Grohl is obvious.