That's a great question! I will have to say that my favorite non-Chan character would probably be Gravelle, played by Boris Karloff in "Charlie Chan at the Opera." Another one that has grown on me over the years is Sheriff Mack in "The Jade Mask" probably because of his dry humor and the way he goes along with Charlie Chan without trying to assert his local authority as other officials have in other Chan adventures.
Thank you, again...so much...for the fun question, Lin!