I've been privileged to attend lectures by some very well-known mathematicians, but I still think that my favourites are all at Cambridge. Maybe it's just because I know them better.
Piers Bursill-Hall (History of Mathematics). Without a doubt the finest lecturer I've seen. He barely has to prepare for his lectures at all - he knows his subject so well that can just turn up and rant for an hour and a half. I'm giving a couple of lectures as part of his course next term, and I'm so psyched about it you wouldn't believe.
Tim Gowers (Foundations). I've not seen anyone else explain a complex and often unintuitive subject with such consummate ease. He has a natural talent for distilling an unfamiliar idea into everyday language.
David Tong (Classical Mechanics/Theoretical Physics). One of the most animated lecturers I've known. He never gives a dull lecture, and he goes to extraordinary lengths to make tough ideas seem simple.