Of those I've seen, I really enjoyed Robert Picardo and Michael Dorn, who both had some terrific stories about their acting career.
Picardo had a hilarious first-person tale about the Doctor getting genitalia and an equally hilarious story about working on "Gremlins 2". I got signed photo from him of the "Johnny Cab" robot from "Total Recall". I asked if it was sculpted based on his likeness, and I think said it was. Apparently he also liked Schwarzeneggar as a good governor (not being from California, I have no idea how popular he was in office).
I asked Michael about which guest stars really made an impression on him, he sid Jon Colicos and John Anderson from "Survivors".
I also liked meeting Gerrit Graham (Quinn), though I asked him more about "Gargoyles". He actually didn't remember his role in it that much. He thought he played a gargoyle! (To be fair, for him, it was a few lines in a recording studio 15 years before). He was cool about it, signed my DVD of the show, and even sounded curious about where to get it. (In retrospect, he might've been humoring me).
The Q&A with Ethan Phillips was okay, I guess, but nowhere near the above two. The dual one with Nana Visitor and Chase Masterson wasn't bad, but now I don't think I'll be able to think of Chase without remembering that depressing stalker/lawsuit story.
Jonathan Frakes and Shatner were both in town last year. Unfortunately I got the schedule mised up and missed out on the Frakes Q&A (which is too bad, since he's always struck me as fun guy).
The Shatner one was something like $25 to get into the Q&A itself, which REALLY bugged me! Didn't go to that.
Probably the least interesting was Bruce Hyde (Kevin Riley) about 14-15 years ago. I feel bad saying that because he was actually a really nice guy who remembered my name, but kept talking about a bunch of subjects people really weren't interested in.