I'm fortunate, in that I've had the opportunity to see many clinics over the years. Live long enough, and things start adding up for you.
Let's see...some of the more memorable ones...
Dave Weckl at the Connecticut Percussive Arts Society Chapter "Day Of Percussion" in 1984. If I remember correctly, this was one of his first clinic appearances. I don't remember exactly what he talked about, but I remember being impressed by how great his drums sounded, purely acoustically (no PA, nothin').
Ed Thigpen giving a clinic on (what else?) brushes. He was wonderful - a true gentleman, quite gracious - but it was really humbling to be in the presence of a master brush player like that. It made me realize how rudimentary my own brush skills were (and are, unfortunately).
Louis Bellson. A legend, a phenomenal player, and everything you hear is true - he has to be one of the absolute nicest guys on the planet.
Dom Famularo at the CT PAS "Day of Percussion" a few years ago. Again, believe the press clippings. Not only is he a killer player, but his spirit and positive energy are remarkable.
Airto, at yet another CT PAS "Day" in 2004. He split his clinic between "percussion" and drum set, and sounded great at both. He's one of the most musical drum set players I've ever heard.