Sirtis I've seen a lot on conventions. For some reason her and Dorn always seemed to find time to go East Coast conventions so they showed up a lot at them. Both were always fun. I've seen each at no less then 4 conventions a piece.
Brent Spiner is one I feel like I had the privilege of seeing on stage. He did a stand up comedy routine that I remember had everyone roaring, but I was far too young to remember anything else about it. He was EASILY the funnest actor I've gotten to see at a convention.