The most pleasant "celebs" I have met to date are:
Craig Parker (LotR/Legend of the Seeker) : Funny, sweet and an all-around great guy!
Mark Singer (V/Beast Master) : Will go out of his way to shake your hand and tell you how great your costumes are. My boyfriend and I were walking around the Dragon*Con Hall of Fame a few years ago in our 'Pirates of the Carribbean: Dead Man's Chest' costumes, and my boyfriend decided to go talk to him, since he's a huge original 'V' fan. He got up, walked around his table and just started shaking our hands, profusely. Saying: "Oh movie quality costumes guys! This is unbelievable! I love seeing all you guys in your costumes!"
Doug Jones (Abe Sabien/Pan from 'Pan's Labyrinth', etc): He's a big hugger! He loves to bring everyone in for a hug...he loves to wrap his HUGE hands around your head though

. He hung out with us for a LONG time, Sunday night of Dragon*con 2009. We were having an informal party in the Marriott lobby, and he and his manager just popped a squat with us, since he knew a friend of ours, and just started talking!
Sean Astin (Samwise, 'Goonies') : Went out of his way to talk to a friend of mine that has ALS. He was sweet, calm and funny. His handler at Dragon*con 08, was not -_-. Total beotch. But she was also the handler for a few Harry Potter guests, in previous years, and I've heard horror stories about her. Sean Astin wanted to give my friend with ALS a free photo, and the girl turned, gave him an evil look and mouthed "get the money." Evil.
Morena Baccarin - ('V', 'Firefly') : Really sweet! Saw my boyfriend and I dressed as Belle and Prince Adam from 'Beauty and the Beast' and exclaimed: "THAT IS SO CUTE! I love Beauty and the Beast!' and waved to us.
I've run into a few jerks as well. John Noble from 'Fringe' and LotR is a perv. I've heard stories about him grabbing girl's butts and other parts -_-. The guy that plays Lex Luthor on 'Smallville'...totally forgot his name...he's not very friendly at all. Especially if you ask for a pic when he walks by at a con. Very rude. Lou Ferrigno is always at Dragon*con, sitting in a big chair in the Hall of Fame, looking like he owns the damn place. He's inconsiderate and rude. There's been countless other rude celebs, I just can't remember what their names were, and for good reason -_-.