I'd have to say I like Anthony Kiedis's vocals, although if I'm going to be honest, it hasn't really been technically good until this latest release. His singing in RHCP's early career sucked. I still love it though.
I also like Serj Tankian's vocals. Hard to describe why, seeing as how he's not really that fantastic by most standards.
Then there's Michael Jackson. What can I say? I have a lot of respect for the man. I mean, his personal life really doesn't matter to me.