Call me fickle, but I can never have just one favorite of anything. (don't ask my views about monogamy, lol)
Among my favorites are Julia (if I had to name just one favorite, it would be her), Liz (especially pre-1968 Liz), the freres Jennings (hubba hubba!), Magda, any character played by Thayer David (but especially Professor Stokes, Count Petofi, and PT 1970 Stokes), Hoffman the housekeeper (I love watching her torture Maggie, yes I am a certified sick puppy), and the original 1897 Quentin Collins (especially pre-curse).
Cassandra Collins deserves, nay, DEMANDS her own category. Those dresses! Those death-glares! Those WIGS!
There are lots of other favorite characters--1966 Carolyn, Pansity Trask, Madam Findlay, Bathia Mapes...
Oh and CLAUDE NORTH!
G.