Ah man, I wish I'd started posting to this thread earlier. I have so much to say. I actually like singers who have "bad" voices (Corgan, Geddy Lee), as long as they're interesting and unique. The ones I can't stand are the people who have bad voices and then try to sound like everyone else who has the same bad voice (Nickleback, Puddle of Dum, etc.)
As for worst live singer (remember? that was one of the stipulations at the beginning of the thread.) I would probably say Raine Maida (sp?) from Our Lady Peace. They're one of my favorite bands ever, but dam! That man cannot sing live.
And since we've named most of them anyway (Geddy, Raine, Celine, Nickleback), I nominate all the rest of the Canadian singers who we've forgotten: Shania Twain, Sum 41, and Arrogant Worms.