I love Emilia Clarke too, but if you search "bad acting gif" in google images (which I've done before to find funny reaction gifs) you'll see a lot of them are from her for some reason.
She's pretty polarising. Half the people love her because they love her character, Khaleesi. Others (often those who've read the books) think she's poor actress who isn't bringing enough to such an awesome character. I love her though. Then again, I haven't read the books.
I don't think I can choose a worst actress in a show that depicts women so incredibly.
If I had to though, probably the woman who plays Olenna Tyrell. She overacts, which is quite common for a woman of her generation.
She's still amazing though.