Could anyone recommend some films where Stack played a villain?

Caddyshack II. He played Chandler Young, the man who was hellbent on destroying all of Jackie Mason and Chevy Chase's fun.... :)

And on a similar note: "Airplane" hilarious flick

First time I saw him (Stack), or should I say heard his voice...was about twenty years ago when he was the voice of Ultra Magnus in Transformers: The Movie.

I can remember that like it was yesterday...a Saturday in early August 1986.