Ryunosuke Kamiki - My ultimate favorite. He plays a variety of characters and he knows how to live those characters. I love his voice, too!
Eita - He plays various characters as well and he's always believable in each of them (so far with the list of his dramas that I've seen).
I still need to watch more of the following ones and they're probably going to my faves:(I've seen him only in...)
Matsuda Ryuhei (Mahoro Ekimae Bangaichi, The Great Passage)
Sato Takeru (RurouKen movies, Kanojo wa Uso wo Aishisugiteru, Beck, Bloody Monday)
Sometani Shota (Wood Job!, As the Gods Will, The Snow White Murder Case)
Masataka Kubota (Saikou no Rikon, N no Tame ni, Rurouni Kenshin, Kanojo wa Uso wo Aishisugiteru)
and I have a lot more to stalk. :))