For me it was Jeff Kinney. I was meeting a different author I love, but there wasn't enough room downstairs and we had to go up. Turns out that's exactly where Jeff Kinney works! He's the author who got me into reading so I thought that was so cool
I met Craig Ferguson a few years ago, I was just a stand-in contestant for a game show pilot he was filming. He was super nice, took the time to say hello and have a quick chat before getting down to business.
It was really cool to watch him, he was so calm and cool until the camera started rolling and then he was just 'on'.