Ok my current absolute favorite seiyuu, and I am about halfway through going back through everything she's voiced in...
Princess of moe, Hanezawa Kana. I can listen to her voice all day, especially loved her in bakemonogatari and currently in nisemonogatari. But her absolute best so far (as she wasnt just being cute but sad/serious/ect) was Darker than Black: Ryuusei no Gemini. I doubt I'd have even liked Gemini that much if she hadn't voiced Suou so well. The end scene with Suou was so freaking sad, she voiced it so good. Hope to see her in more serious/varied roles like DtB and Canaan.
Kugimiya Rie still in second, she can pretty much tie for first really, considering shes the star of 2 of my favorite airing series (zero no tsukaima and shakugan no shana, not to mention shes in fairy tail too, busy busy!), Hirano Aya third (really love her as lucy). I can't remember his name (looked it up, Hayami Sho), but I also really like the guy who voiced Aizen and currently Sakai Yuji/Sairei no Hebi in Shakugan no Shana III and the parfuum dude (lol) in fairy tail. Just sounds so badass, usually.
And Kayano Ai, she's relatively new and hasn't been in much yet, but I really like her in Guilty Crown as Inori. I expect more good stuff from her hopefully as Inori is pretty well liked.