This is a tough category for me - I'm really not much of a singer, but there are people who really make me wish I could. Part of the issue is separating pure vocal ability from that frontman persona.
There are numerous examples of powerful singers who make me stand up and take notice - David Bowie, Meat Loaf, Sarah McLachlan, Jill Sobule, Roger Daltrey, Mike Keneally, Adele. But for pure vocal ability, these are the ones.
Kate Bush. She's one of those artists that just seems to have no artistic fear and has done so many different things with her voice. She'll go from powerful belting to an almost novelty-type screech to a character voice. So many classic performances, but one of the most impressive to me was the album she did where she used the Trio Bulgarka. They have a way of singing with no vibrato, and she learned to do that and did harmonies like them so well that if you don't look at the credits, you can't tell which ones are her and which are the Trio.
This is one of my all-time favorites of hers. If you've seen the movie "She's Having A Baby", the use of this song is perhaps a bit cliched, but powerful. The video somewhat apes that scene. I wish she'd release her videos on DVD.
But I'm not sure they come much more inspiring or better than Freddie Mercury. I'm not sure I've heard a better singer than him. Gone way, way too soon. But what a voice. This may not be the best example, but it is part of what is possibly the most powerful performance I've ever seen - for a guy who was told he probably shouldn't sing that day, nobody came close to Queen in terms of owning the stage that day. I still get chills watching the crowd do the clap like in the video.