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.
yes, I've seen him a few times and it always seems like he's mainly going for a laugh. He'll tell a joke rather than answer a question, but it's still very fun
the BEST is when he's at a convention that Patrick Stewart is at. When Sir Stewart is on stage, Brent will sneak onto the question line and when it gets to him he uses this nasal voice and acts like someone asking rediculous questions, so funny!