Please add to this list:

Uncle Billy - only two appearances but often talked about. Ward's eccentric uncle.

Capt. Drake - the captain for Eddie's alleged Alaska fishing summer. Really funny scene when he interviews Eddie.

Angela Valentine - the most infrequent guest, the back of her head is briefly seen once, and she has no lines. But she's talked about quite a bit.

Have to give Howard McNear a mention for "Andy The Barber" here.
I always preferred Karl Swenson's "George Haskell" over others

Jackie Waters? Only kidding

Yes Karl Swenson was very funny in Voodoo Magic.

I thought Judge Henderson was rather funny.

Captain Drake gets my vote.

Yeah, Jackie Waters. Or maybe David Manning.

Come to think of it, David was kind of funny. His mother is on the phone and he's gnawing on his fingernails.