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
Dizzy Gillespie. Way back in high school our school jazz band opened for Dizzy at a festival/contest. I was heading to the rehearsal room and saw our director chatting with him, and the director waved me over and introduced me. He said “Hey man, it’s cool to meet you” and shook my hand.