Michael Kiske. His voice makes me want to step on babies' heads. But part of it also is the lyrics he happens to sing, which isn't entirely his fault since he didn't write them (I think). I actually prefer Kai Hansen as a vocalist, but that could have something to do with the fact that Walls of Jericho was more of an old-school heavy metal album.
In addition to his annoying voice, Michael Kiske also sucks for renouncing metal a couple years ago. He made some big statement in which he said something to the effect of "I"m done with this pathetic scene and its puny morals." Yes, that's right, "puny morals". And this is coming from the signer of the happiest, fluffy-pink-bunnies-est band in metal history. He should be forcibly drafted into Belphegor.
Runner-up: the guy from Himsa (which is a legitimate band on the basis of their cool instrumentation, their hardcore-esque vocalist is the only thing I don't like).