I think all the suggestions have good points but I would like to throw in another couple in for the witches.
June Brown as Granny Weatherwax. I know she is now a regular in East Enders (A tv soap here in the UK) but, despite this, her acting is really very good *ducks from the stones thrown by East Enders fans*
(she was also Nanny Slagg in Gormenghast).
Miriam Margolyes as Nanny Ogg. She was in a couple of the Harry Potter films but don't let that put you off. I have seen her on a tv chat show and I thought she was so made for the part. (See the abbreviated clip here if you want a giggle)
Definitely agree with Brian Blessed for Ridcully though!