Some of the best known for playing "nut cases" in their films include Johnny Depp, Brad Pitt, Jack Nicholson, James Cagney, Anthony Hopkins and Jim Carrey among others. They usually play the roles of Villains, Evil Wizards, Thugs, Nut Cases and entirely Meanie. Check out http://www.imdb.com/list/wSSLi3pH5vA/ for more information.
Christopher Lloyd comes to mind. He played a mental patient in "One Flew Over the Cuckoo's Nest", burned-out ex-hippie Jim Ignatowski on the sitcom "Taxi", and the loveably bizarre Doc Brown in "Back to the Future".
That freak who was in the remake of "Ben". He was the dad in back to the future. He was on letterman and freaked out and almost kicked Dave in the head. Dave cut to commercial and threw the guy out. He was gone when the commercial was over. Think the clip is on YouTube.