I don't actually find any of the vocies of the classic characters to be ugly, but that might have to do with the fact that I'm so used to all of them. Generally speaking I tend to have a problem with voices that sound really strained and forced, so if I hadn't grown up with them, I have a feeling I would have had a problem with for instance Barney's and Marvin Monroe's voices.
There are however a few voices in the show that has started to annoy me, not because the voices are "ugly", but because they have recently strated popping up absolutely everywhere in the show. I'm thinking of that guy who does all the black male characters recently, as well as the old guy with the german accent. They're both actually really interesting voices but my God just give them a rest already. I've also gotten really tired of Tress Mcneille's regular "buisiness woman" voice since it likewise is used on so many characters and always sound exactly the same.