Jonathan Carpenter from The Contortionist. Not really for his actual vocals, but for the patterns he does and he knows when to keep things instrumental. Awesome style. Phil Bozeman from Whitechapel Jonny Craig from Emarosa/Dance Gavin Dance if he would stop being a tool. Tommy Rogers from Between the Buried and Me. Just overall perfect IMO.
Chris Baretto, theres a video of him doing vox in studio for Haunted Shores way back, and his voice by it's self just sounds unbelievably raw. Instant fanboy.
Elliot Coleman is also an amazing clean singer, though I'd hate to sound like a bandwagon fan hahaha.
John Carpenter does have an honorable mention he influences me a lot.
I'm sure there are quite a few clean singers than I'm missing, but they escape me.
EDIT: HOLY SHIT HOW COULD I FORGET ABOUT CHRIS PREECE????? ALLSTAR!!!
Jeff Ocampo of The Kymatica (local band from Tucson, AZ, check 'em out) Casey Crescenzo Demon Carcass Anthony Green Jon Mess Kurt Travis Spencer Sotelo Jake Veredika Andy "Dozer" Atkins Mike Lessard Hayato Imanishi I have a lot more favorites, but I don't want to clog up the thread.
Metal/Rock: Rob Halford Sebastian Bach Layne Staley Tairrie B Phil Anselmo Mandy Lion David Coverdale
Other: D'Angelo (when will you record another album, damn you!!) Prince Pink Annie Lennox Adele Sting Tom Jones Keith Sweat Amy Winehouse Lynden David Hall (RIP) Sade