Well I got some negs about Spiner, suffice to say he put me off so much I didn't bother with an autograph.
LOVED Picardo, he came out on stage, set down some speakers and launched into his "Tuvok I understand.." song. It was gold. And he was funny and kind and nice and lovely when we had photos.
Huh, what negative things did you hear about Spiner? He's always one I've heard from others as a must see at conventions.
Many of the Trek actors are neat to see, but few actually garner that "Must see" because of sheer awesomeness in their presentation. I certainly think everything I've heard about Picardo makes me think he's a must see on stage (especially the conventions I've seen him at on Youtube).
Most the other actors? They get into some really interesting stories about happenings behind the scenes, but they don't entertain at the level of Picardo or Spiner. With some of the antics Wil Wheaton has been involved in lately (IE Denver Comic Con and the Flash Mob Dr. Who Incident), its almost hard to want to miss a convention he's attending too.
There's probably others who fit into that category, but none spring to mind.
LeVar Burton I will say is always a great convention to take kids to. He goes way out of his way to do something special for the kids.
Frakes is a better conversationalist by a long stretch. Conventions just never do him justice. One summer back around the time I was going into college (around 2000), he conducted a 3 day seminar at PSU (his almatar) discussing the future of science. That was a fun, fun, fun event and not terribly expensive either, especially for what you got. Though Star Trek wasn't the main topic, it came up a lot, but only in the nerdiest ways possible and it was great =). He talked a lot about how Star Trek tried to act as a bridge for modern science and education and where some of the theories came from.