So many to choose from: Tate, Robinson, Morell, Donlevy, Keir, Mills and Flemyng. I actually really liked Jason Flemyng in the 2005 BBC live remake of the Quatermass Experiment which was a fun project to watch.
However I'm going to go back to the classic serial and to an actor who also played a terrific villain in 1984. And he was Watson to Peter Cushing's Sherlock Holmes in The Hound of the Baskervilles.
My favourite Professor Quatermass is Andre Morell.