Wow, finally an interesting question on here. It's a tough call because he was in so many great movies and I was very upset when I learned months later that he died.
I'd have to say my favorite was THE FRENCH CONNECTION, by a great director William Friedkin. Next was ALL THAT JAZZ from 1979. Of course JAWS.
My favorite final performance of his was on the show Law & Order: Criminal Intent, a 2007 episode titled "Endgame." He played an inmate on death row, and right before his execution he directs Det. Goren to a scrapbook buried in an old attic. He had taken pictures of women he was with over the years, some of them his victims, but one photo stood out to Goren. It looked very similar to his mother. Goren always felt that he & his brother were very different, and he remembered a man that was a friend of the family when he was little. It was a great episode, and you should look for it.