I always thought Roy Khan from Kamelot had one of the best voices in the metal/rock world. He really puts Kamelot a step higher than your average power metal band. He's good on all songs but really shines on the slower ballad types like these 2 :
Abandoned:
YouTube - Kamelot - Abandoned
Don't You Cry:
YouTube - Don't you cry - Kamelot
You don't need to be anyway a fan of metal or even rock to enjoy those 2 songs so please don't hesitate if you're worried about not liking someone in that type of band
On a completely different angle i really like Fiona Apples voice, she's one of my favorite female vocalists. Such a smooth relaxing voice and tends to sing songs with great lyrics which always helps. My favorite song by her has always been Never is a Promise.
Never is a Promise:
YouTube - Fiona Apple - Never Is A Promise
Another set of singers i really love is Los Lonely Boys, all three of them can sing (as evidenced by them often bursting into 3 part harmony in concert). They also churn some seriously catchy light blues rock tunes, i'd highly recommend checking out any of their 3 albums though their debut is still my favourite, they are a really feel good band perfect for summer. Another note on them is that i don't think the songs they release as singles (generally the one's that will prob be the biggest commercial hit granted) represent their best material.
Los Lonely Boys Diamonds:
YouTube - Los Lonely Boys - Diamonds
I also really dig musicals and there's literally hundreds of quality songs with amazing vocals in that department. I've always ranked phantom of the opera among my favorite so any of the songs off that is worth checking out for amazing vocals. I know a lot of people don't like the vocals in the movie but i think Gerald Butler does an amazing job considering he didn't really have any singing experience, but then again i've always favored passion in my vocals over technical prefection
See what you think yourself:
The Phantom of the Opera (Movie Version):
YouTube - Phantom of the Opera
I could literally go on for days about my favorite vocalists that barely scratching the surface and if i had replied an hour ago or an hour later a whole different group of vocalists would prob have sprang to mind