I don't like picking favourites but in terms of the vocalists I enjoy, I think it comes down to ability + how recognisable they are. I'll second Jens, he's a bit of a one-trick pony, but what I find so interesting with him is the delivery pattern of his vocals - something that's missed by some people. Akerfeldt... I seem to think of the guy in past tense for some reason - I guess it's because Opeth are getting so un-metal (not a bad thing, just different... haha okay I miss the death vox) - at his peak he was immense though.
Devin Townsend obviously - such a versatile, instantly recognisable voice - this vid sums it up:
There's others like Ihsahn, Chino from Deftones, Warrel Dane in Nevermore's best years as well.