Right now my #1 favorite Japanese actor is Kubota Masataka.
He's incredibly versatile. He picks very different roles and is capable of really transforming himself.
His career improved a lot after "N no tame ni" (won several awards, including Best Drama of the Year) so he's been extremely active. I actually feel he should be more picky about his projects nowadays.
What I hope/wish for him now is that he's cast as a the lead in a high-profile romance drama. He would make a great romance hero.
Sakai Masato without any doubt at all. His acting in Hanzawa Naoki & Legal High totally carried both shows and he is probably the single reason why both of these shows are in my Top 5 favorites.