Joni Mitchell wrote the song and her version is my favorite at the moment. Judy Collins made it famous. I am on a Dave van Ronk kick these days and his rendition, though less 'accessible,' is a haunting contender.
According to the Wikipedia entry on van Ronk, "Joni Mitchell often said that his rendition of her song "Both Sides Now" (which he called "Clouds") was the finest ever."