Mustrum Ridcully
Sadly miscast here. Joss Ackland is a great actor, but far too gentle and far too gentlemanly. Ridcully is an comic old buffer, yes, but he's no fool, he's a powerful wizard in his own way and he ought to be quite frightening.
It was clear to me from the moment Ridcully was introduced in the books that he could only be played by Brian Blessed. With a mighty beard - someone describes him somewhere as looking like a man halfway through swallowing a cat