Lee Min Ho- Although I gave him a few second chances, he is still no.1 on my black list 
Park Shin Hye
Go Ah Ra
- she's so loud and annoying
Hwang Jung Eum- she's so loud and annoying x2 (and her voice is unbearable)

Kim Soo Hyun- Her acting is mature and she does great as a sassy bitchy girl but I don't really like her in other kinds of roles. I'm actually not happy with her acting in Ruler: Master of the Mask later this year, since I've been looking forward to Sung Ho's next drama and I want to fully enjoy it. :'(  
Jun Ji Hyun

Nam Goong Min idk what it is about him... if it's the characters he plays or the way he portrays them but I just find myself steering away from dramas where he's the lead and ultimately, I dropped BGS because of his character... although I did enjoy his little cameo on Doctors... I thought it was very poignant 
Lee Min Ho - his acting is so limited, not only his role choices. I have the same issue with Park Shin Hye. at least she has a warmth about her that I can't hate and it's possible to finish a drama with her as a lead. she leaves no impression but I don't particularly dislike her like Lee Min Ho  
Lee Min Ho - it's not that I don't like him, it is just he is so overrated (especially in my country), and I think there are so much more actors that can do better than him. 
Park Shin Hye - she's a good actress, but I really don't know why I can't finish any of her dramas.

For me it's Park Shin Hye (she can't act worth shit) and Ha Ji Won (always looks like a victim of a childhood trauma even if she's not playing one) / Lee Seung Ki (I can't connect with him at ALL) and recently Lee min Ho (I used to like him at first) But recently I can't stand to watch anything he's in. Hwang Jang eum (I just wish she'd stop exaggerating and screaming and playing the angelic, unrealistic female lead all the time!)