Based on dramas, I'd say:
Kim Jae Won
They are both very strong actors and I will watch anything with them. Unless it's sad stories or maybe I should watch to 'Can You Hear My Heart'? I really don't like sad stories anymore.
If we counted movies too, I'd say Cha Tae-hyun.
As for favorite actresses, I'd say:
Jung Da Bin
Moon Geun Young
I love these two actresses. Very stable actresses who always delivers. I like that they aren't typically pretty like so many actresses. I can't seem to focus on the story and acting when the person is just too pretty. It's really weird to see that MGY doesn't portray little girls roles anymore, but more womanly roles. It's really great to see that now.
I really like how she looks and how her voice sounds. (by voice I don't mean singing, just voice)