Not sure about the best of all time, but cetainly very, very good.
SleepingLion, I don't agree that he is only good in those two roles. They are so different from each other that it is not too much of a stretch to believe that he would be great in other stuff as well. Indeed, I think he's really good in 'Peter's Friends' and remember my 16 year-old self being very indignant at people who kept saying he could only play 'upper-class twits' around the time that film came out and felt that it was testament to his potential. (I was a bit odd at 16
.)
I think he has had two careers, really. I bet if he had started out as a 'straight' actor he would have done a lot more roles and we would have more evidence of his range. Instead, he started out in comedy so has had less time to prove himself as an actor. I'm very glad about this personally. Where would we be without 'thicky' Prince George and all the fabulous Fry and Laurie sketches?
Phew, I think I've been waiting 20 years to get this post out of my system!