Personally, after having listened to the audiobooks I have trouble seeing anyone other than Nigel Planer in the part.
I felt exactly the same way after I saw him playing Sydney in Hogfather; he looked almost exactly like Rincewind does in my mind, only with a different coloured beard. Every Discworld fan I've talked to about this agreed that he would have been the perfect Rincewind, but David Jason was a producer and gave himself the role.
My main problem with TCOM was the lack of a spark between Rincewind and TwoFlower. They had a great love/hate relationship in the novels, but I never felt that come across properly in the adaption and I feel that it is primarily Sean Astin's fault. Contrary to this, I thought that the Albert/Death relationship came across very well in the Hogfather adaption and it is one of the reasons I like it.
I think that David Jason would be good as Groat in Going Postal, but I will be seriously pissed if he decides to give himself the role of Vetinari!