He played John Wayne Gacy in "TO Catch A Killer", which I thought was pretty decent for the early 90s.

I thought he was really good in the role, bringing a menace to the character that was wonderfully creepy.
John Gacy, though, was actually 7 inches shorter than Dennehy. It would have been more interesting to
see how a little twerp was able manipulate dozens of young men and the police without towering over them.