David Tennant for Rincewind!
Well, everyone else here has an opinion, so heres mine.
Tennant would have been excellent I reckon. He's a great actor, can do manic/petrified/running away, and is a more suitable age.
I think David Jason looked the part pretty well (from the Kirby drawings anyway) but just didnt play it right. He wasnt scared, sarcastic or inept. Just kind of bored and confused.
My 2 cents.
Thank you.