The witches are my favourite characters so far (although I do love the Watch books as well). Not sure why I like the witches, but I do and I'm desperate for TP to write more books involving them. Especially Granny.
Equal rites is probably my least favourite of the witch books, Carpe Jugulum being my favourite (unless we're counting the Tiffany books...in which case A hat full of sky is my fav).
I agree that Tiffany seems to be in line to take Granny's place (although not for some time I hope :? ), she reminded me so much of Granny as I was reading Wee Free Men that I was glad when Granny made an appearance at the end of it.

Anyhoo...as a whole, I think they're fantastic. I both admire and fear all of the witches. Those books are the most frequently read in my collection and Nanny's cookbook is a frightening curiosity in my kitchen. :p

Cass.