Of all the Star Trek actors I've met, two really stand out as being the best as far as experiences go.
One was with Avery Brooks, who is so full of life and seemed to be having an amazing time meeting fans. When I got up to him he pulled me close and was just a warm, engaging guy and easy to talk to.
Connor Trinneer was also great. I met him earlier that day, and that night he showed up at the bar to hang out with the fans. He was really great with my dad, who came with me to the convention but never got to meet any of the actors until then since he didn't want to buy any autographs or photo ops.
As far as non-Trek related people goes, my best experience might be with Kristen Bell. She was very lovely and down to earth.