Like most on here, Shatner was probably the biggest let down when you meet him in person. On television, during his convention talks, and in his books, he goes on and on about how he loves the fans. However, when you do get that one on one with him, zero interaction. I had been wanting to meet him for years. When I got my photo op with him, he never turned in his seat to look at you. Stone cold expression on his face in the pic. When I got my autographs, his assistant took your item from you, slid it to him, he signed it, and slid it away. No hello or anything. Only in it for the $.
Spiner, I agree with the comments here. His constant sarcasm makes him come off as a jerk. I had a cast picture where all the cast up to that point had dedicated the photo to me (Stephen). When I asked him to do the same, he stated, "Because, just what the photo needs is another "To Stephen." I wanted to say, for $30 be glad I'm not asking for "Ode to Spot."
Dorn - another that is there for the $. When I got an autograph from him at Dragon*Con last year he was sitting there watching a basketball game on his iPad. He had absolutely no line, so it wasn't like he was busy. Never took his eyes off the iPad screen to acknowledge me in any way. Just took my $ and signed his name.
Nichols - Nice lady, but can be a diva at times.
Beltran - another one that shows up for the $. He will acknowledge you, but that's about it.
Best to meet in person? George Takei is the all time great. Super, super nice guy. Avery Brooks will shake your hand and talk to you for a few minutes. Never feel rushed with him. Met Gene Roddenberry about a year before he died. Went on a TrekCruise in 1989 that he was on, and I literally ran into him coming around a corner. I was completely flustered, but he stopped and talked to me for a while. Later, I encountered him in the bar and he signed a picture for me.