Of the thousands and thousands of terrible actors out there, I can't believe people would mention the likes of Depp, Cage and even Cruise (who has at least been solid in a handful of movies). But.... I know it says "your top 5", so I'll get over it.
My list:
Mariah Carey - Starred in Glitter.... god it was awful.
Britney Spears - Another one time actress, incidentally her 'one-time' was just as bad Carey's.
Hilary Duff - Not quite at the Spears/Carey level of suckiness, but the way she just churns out one pile of sh*t after another, it's hard not to label her as one of the worst ever.
*daylight*
Steve Martin - Decent standup comic in his day; terrible actor. From the recent remake of the Sellers' classic, The Pink Panther, to pretty much every other decent concept that has been ruined by the man whose range varies from bored to slightly drowsy. I wish he'd stop making movies.