These are the actresses who I enjoy watching on screen: most of them have a certain on screen quality that just draws you in and often they're quite versatile. (Or they've been in a toku show ;-p) Is a bit biased towards the younger gen as I've only really watched dramas from the last 10 years or so.
Aoi Morikawa
Mei Nagano
Kyoko Yoshine
Juri Ueno
Haru
Ruka Matsuda
Ko Shibasaki
Nanako Matsushima
Rio Uchida
Marie Iitoyo
I'm sure I'll add to this as I watch more and more.