Which of his

The Changeling

Hardcore (1979)

The New Centurions (1972)

Anatomy of a Murder and Petulia.

I thought The Savage Is Loose was a pretty good movie, and worth checking out. His wife at the time, Trish Van Devere, is the co-star. Rather controversial for the time, and I’d imagine even today as well.

Just a heads up though for anyone wanting to see it; you won’t want to watch it with your mother :D

https://www.imdb.com/title/tt0072114/?ref_=nm_flmg_act_43

Petulia is a terrific, offbeat film with a great performance by Scott.

I liked the Hospital a lot.

Rage (1972)
[ https://www.imdb.com/title/tt0069158/?ref_=nv_sr_srsg_0 ]
To be fair he was extremely versatile so picking a favorite role will lead one all over the place.
With that in mind I would also like to include
They Might Be Giants (1971).
[ https://www.imdb.com/title/tt0067848/?ref_=nm_flmg_act_49 ]
Dr. Strangelove (1964)
[ https://www.imdb.com/title/tt0057012/?ref_=nm_knf_t4 ]
The Hindenburg (1975)
[ https://www.imdb.com/title/tt0073113/?ref_=nm_flmg_act_39 ]