I've had a few. Most recently I took lecture courses by Robin Knight and Ehud Hrushovski.
Both absolutely stellar lecturers, the hours just slid by.
Robin Knight has an almost conversational way of lecturing, and really invites you to try to catch him out. He focuses on making sure you understand why every assumption we make is necessary.
Can't quite pinpoint why Hrushovski was such a good lecturer (and class tutor, happened to take my classes too.) However I felt like I got everything I needed, I could ask any question I wanted, and had a good time in the course.
Have also had classes with Joel Hamkins, he was a brilliant tutor.
I've also had some, ahem, not so good lecturers. Infamously bad at lecturing, Kevin McGerty. He would deliberately swap up the font size and margins on his notes away from the standard to make them harder to read.
Trying to think if there have been any real famous ones (like not just huge names in their area of study.) Can't think of any, our department's most famous names (Andrew Wiles, Roger Penrose etc) don't interact with undergraduates.
A good lecturer ensures her notes are readable. She knows how to engage her audience, and read them to make sure they're understanding what's she is communicating. She sets problems that are challenging, but give clear purpose, so the student can see what exactly they will learn by completing them.
