Now that I've watched some more japanese shows and movies I'd have to say one of my top favorite j-actresses is Kiritani Mirei. She is just great in Arakawa under the Bridge and HUNTER.
And as many have said already, Toda Erika is very good.
Kitagawa Keiko is my most favorite J-Actress coz we have the same name ^^ hahaha, but mainly because she is a GREAT actress followed by Kutsuna Shiori and Shida Mirai
for J-Actor, my most favorite is Sato Takeru ^^ followed by Tegoshi Yuya and Osamu Mukai