At the start of Madoka Magica it seemed like Homura had no emotion, but later on she expresses some.
Mami is killed= poker face Sayaka commits suicide= breifly cries a bit when Madoka won't relize something before going back to poker faceness. Kyoko goes to die with Sayaka= poker face Madoka is in danger= OH FUCK NO I BETTER USE MY TIME POWERS TO SAVE HERRRRR
Rei Ayanami. Can't believe no one 投稿されました her yet. She the archetype of the emotionless girl. Even Yuki Nagato is considered によって some as a clone of her.
I'm going with the obvious and saying Mytho from Princess tutu. I know he's supposed to be emotionless but, even when he gets his emotions back the 書く and voice actor still sounds uninterested.