Robin Williams. Certainly from the "early, funny days", before someone helped him look up "pathos" in the dictionary and we all had to reach for the sick bucket.
And although Patch Adams et al are mind-bogglingly shite, he HAS had one or two good parts since, not least of all playing a pretty good psycho in Insomnia. I also find it slightly ironic that his only decent comedy film since Good Morning Vietnam was playing a role written for him by another has-been comedian whose "early, funny days" are long since past, i.e. "out of focus" Mel in Woody Allen's Deconstructing Harry...
Mork or Mandy.
I'll choose Mork. When he was drugged out of mind and happy he had reason to be caught and lost and inventive. Then he straightened and cleaned to the robotic liner of acceptability. Always summer for him since then. An Eastbournian.