Initially, I wasn't going to weigh in with my thoughts of Robin Williams since everyone in the world (and their sister) are, but he really meant something to me, and writing has always helped me process.
Bar none, Robin Williams was my favorite comedian. Eventually he became one of my favorite people. Hilarious doesn't begin to describe his brand of comedy, a deft mix of relevant (but not nasty) social commentary, improvisation, impersonation and relatable observations about family, aging, success and failure (plus the occasional dick joke). I don't think that explanation does him justice, but it's the best I could come up with.