I love the brainy ramble of him, he always seems to be mid-sentence, teasing, feeling his way around to a light slight, breathing on women's shoulders who catch on too soon to his advances. He's built for comedy and he's proved himself there in an effortlessly off the cuff way (The Big Chill, Between the Lines, etc). He doesn't get a lot of credit for stretching tho, and he's tried and failed a lot in plain sight.
The Fly perhaps his career best perf, especially in the way he darkens, creeps and curdles, his initial romantic wit. Quite good from the same year, there's Ray Bradbury Theater's The Town Where No One Got Off. And I like the first ten minutes of Mister Frost where he goes head to head with the great Alan Bates, wielding a sort of chilling confidence. I like him in The Lost World too, close to a pure star perf. I recently watched the horror-Hallmark esque Fathers & Sons where he's all over the place but never uninteresting and reminded me that he's a piano player too and how his hands seem to have a brain of their own and always need to be doing something.
Shoutout to all the great 1970s movies he appears in: Death Wish, California Split, Annie Hall, Body Snatchers, and Remember My Name.