Usually, if the acting is good, I tend to sympathize with the character even if it's not something outstanding.
So I should say Kim Tae Hee. I've only seen her in My Princess, but I couldn't stand her through the whole drama.
Also, Park Shin Hye in You're beautiful got on my nerves.
Personally, I dislike Park Shin Hye. I don't like her acting and especially wall kisses. But despite this I consider her as a really beautiful actress.