"Singers who can't act?" All of them.
I will admit, the only singers I can stand acting in movies are Beyonce and Cher, because they can pull it off pretty good. Britney's acting was atrocious in Crossroads (even if I do like the movie a little... :toofunny), and I haven't seen much of Rihanna's acting. I'll never quite understand why every singer feels the need to jump into acting.