I have only ever met celebs at conventions back when I used to go semi-regularly.
My favourite is Ray Park. My girlfriend at the time was in line to get a Darth Maul poster signed by him, so I was kinda just along for the ride. But he made an effort to engage me in the conversation and make sure I felt included. Really nice guy.
The same goes for Ron Perlman. I was with my sister this time to get her copy of Hellboy Vol 1 signed but he made sure to chat with me as well and get a photo with the three of us.
My least favourite is Doug TenNapel. This was back before it was widely known that he's a hateful biggot, so I was excited to meet the creator of Earthworm Jim and get his signature. He was a major jackass.
In between is Billy Dee Williams. He was wearing sunglasses indoors and was obviously either super hungover or super stoned, but either way he was a chill guy so I can't complain.