Man, he was a staple of 1970s TV even though I couldn’t recall until I read the obit what all I’d seen him in (I think his series Doc is where I’d seen him most).
And I am ashamed to admit I didn’t even think he was still alive.
Unforgettable in The Hospital, perhaps the best movie (though not the most famous) that Paddy Chayefsky ever wrote.
He also played the Old Software that gave Tron access to the I/O port in Tron.
And from one of my favorite movies, The Lost Boys, we get the immortal line:
“One thing about living in Santa Carla I never could stomach, all the damn vampires.”
Well, phooey. 
And the only exchange I remember from Doc:
Kid: Is it true you cut up live people in doctor school?
Doc: Yes.
Kid: How Come?
Doc: The live ones keep jumping off the tables.
Fine actor.
Though he did a lot of TV work, he was at his best on stage. I first saw him on Broadway playing the title role in Hugh Leonard’s play Da’, and he was absolutely remarkable. I come from a very Irish family, and I swear, when I saw Hughes on stage, it was like seeing my grandfather alive again.
He was equally great in a much lesser play, Lanford Wilson’s “Angels Fall.” He played just the sort of Catholic priest G.K. Chesterton idealized.