This certainly is a useful thread. I haven't met anyone yet, but I hope to meet Marina Sirtis at Fedcon this year. She seems like a nice person and I've heard good things about her convention persona.
As Clay said, Marina is total blast and really nice person to boot.
Walter Koenig was the friendliest.
I found the same. We ran into him accidentally before the convention started, and he was just total gentleman and very friendly. He remembered us when we saw him at the signing table later and he reacted almost like we were old buddies of his.