Karloff. I have all his Mr. Wongs and all his US-UK horror films except

The Man who Lived Again
The Man with Nine lives
Abbott and Costello meet Dr. Jekyll and Mr. Hyde
The Haunted Strangler
Corridors of Blood

and all the stuff that hasn't hit DVD yet of course (though that's not much)

Lugosi comes in second with about five fewer films that Boris, then Vincent Price.

But if you could VHS stuff then it's Tom Baker as I have all 41 of his Doctor Who stories and some of his movies.