What Dreams may come looks so fitting given that his wife killed herself in the movie... And he came to safe her. The movie was so intense and beautiful... I loved his performance always and I always thought that he like nobody else could express feelings with his face and gestures.

Fisher KIng

Good Will Hunting, Death to Smoochy, One Hour Photo and his stand-up.

This ^^^

His whole personality was conveyed through his voice.  He made that movie work.

Similar to Good Morning Vietnam.

Bicentennial Man and Mrs.Doubtfire

Hook and Mrs. Doubtfire. Although he's a great actor and he's been in some serious stuff too like Insomniac and an episode of Law and Order SVU.  I haven't ever seen a poor performance from him.

The thing I remember him most from is Hook. His performance in that always stuck with me. Honorable mentions would be Dead Poets Society and of course Mrs. Doubtfire.

It confuses me the way people have brought up the Genie so much. I never really thought of that as a Robin Williams role. I mean doubly so considering he only did the voice in the first movie... after that, all the series and specials were done by Dan Castellaneta, voice of Homer Simpson. Robin was good as the Genie, don't get me wrong, but I never saw the man behind the cartoon there. Yet I've got all sorts of people my age acting like it's his magnum opus now, the Genie is now free from the lamp, I mean come on.

One Hour Photo was the first time I had seen him playing a bad guy and damn did he nail it. So chilling. Reminds me a bit of Jon Lithgow, who I had always seen as a comic actor thanks to his wonderful performance in 3rd Rock From The Sun but he proved to be quite adept at playing the bad guy in Dexter as Trinity.

Genie was very much Robin Williams' character. The role was written with him in mind and once he agreed to do it he did a ton of improv while voicing Genie. Here's an article that might help you see how much Robin had to do with the character: http://insidemovies.ew.com/2014/08/12/robin-williams-aladdin-eric-goldberg/

There's also a video in the article where you get to see him working in the studio (Robin's part starts at about 6:45)

Also have to go with Aladdin here. That was, like, THE Disney movie of my childhood.

Edit: It's also important to note that Williams (and Gilbert Godfrey) being in that movie were, along with what the Simpsons were doing in TV at the time, what helped make voice acting into something that stars did, which was hugely important to American animation that followed (even only a few years later, like Toy Story starring Tim Allen and Tom Hanks, or 10 years after Aladdin, Shrek with Michael Myers, Eddie Murphy, and Cameron Diaz). Stars didn't work in animation, prior to this, unless they were very washed-up (for instance, one of Orson Welles' very last performances was as Unicron in the Transformers movie in the 80s)