Almost definately some guy from way back in the day, or some kind of opera singer. For modern and rock I would have to say Jeff Buckley, RIP.
As for whose I like the best, I would have to say Isaac Brock of Modest Mouse and Ugly Cassanova. His voice, while arguably the worst in the industry (he has a huge lisp), fits the music perfectly. I cannot imagine him singing anything else, nor can I imagine anyone else singing his music. It is the most original, refreshing voice out there today, imo. I can't put him at the best though, because it obviously isn't that. It's just nice to hear something completely different.