Here are my top 5 favourite characters Williams has played. Of course the Genie from Disney's Aladdin is amazing but I'd have a separate list for all his role in animation. He was the only funny part of Happy Feet (terrible film) and as I said, he will always be the Genie.
1. Mrs Doubtfire - 1993
Every child growing up in the 80s/90s would have seen this brilliant and actually sad film. Its about divorce, a man trying to find a job and a father who really wants to spend as much time with his children as possible. Its also about hating Pierce Brosnan from a young age and loving the moment Doubtfire throws fruit at him. Based on British writer Anne Fine's Madam Doubtfire, this really is implanted in everyone's mind when it comes to comedy and men dressing up in drag. And yes, the whole time? The whole time he was dressed as an elderly woman. Dude really did look like an old lady.
2. The Birdcage - 1996
Playing Armand, partner to Nathan Lane's Albert, this could not have been a better paring. These two together made a hilarious play, a brilliant film, to me its all about the two together and their relationship. A gay couple pretending to be a straight couple (Lane in drag) to make a 'good' impression on uptight married couple to their son's girlfriends parents. Screaming, singing, awkward conversations, people pretending to be something they aren't and drag numbers.
3. Good Morning Vietnam - 1987
I saw this film some years ago and this was only having seen Williams in purely comic roles. This was a great story based on truth about radio DJ Adrian Cronauer who broadcasts in wartime Vietnam. It's perfect balance of funny and tragic moments. Williams actually and its not hard to believe, improvised most of his radio broadcasts in the film. Even if you've not seen the film, you will know 'GOOOOOOOOOOOD MORNING VIETNAM!'
4. Jumanji - 1995
This is a classic children's film which is faced with being remade at the moment. Williams is Alan Parrish all grown up after being trapped in the board game for 26 years. Its fantasy mixed with adventure and real terror - they are chased by a lion, gigantic mosquitos and harassed by monkeys and they survive an earthquake and monsoon, all in one day!
5. Death to Smoochy - 2002
Williams plays a washed up bitter children's entertainer and basically, not the nice guy in this story and its brilliant. He's naturally funny and usually plays nice or wise characters but Rainbow Randolph is mean, bitter and twisted. Throughout the film he is trying to frame Smoochy the Rhino, his rival played by Edward Norton. An underrated film, directed by Danny Devito no less. If you've not heard of this, go watch it now!
Another few mentions would be the sort lived sitcom, The Crazy Ones, Hook, World's Greatest Dad and One Hour Photo and I know everyone hates it but I loved it when I was younger. It was like seeing the cartoon jump of the page to make a tacky full functioning Sweet Haven. I loved Popeye and though Williams was brilliant, right down to the over stuffed forearms.
As a parting gift here's the musical trailer: Popeye (1980)