I met Patrick Stewart once. I was doing inventory at a Tower Records (remember those?) and he came in looking for a version of the Brandenberg Concerti.
He was awesome. Very polite. Very affable. I temporary blanked on his name and actually called him "Captain." He responded as though I had made no mistake.
I met a few astronauts. They're not generally very famous, but they're interesting people. I met astronaut Dave Scott while selling him a leather jacket. He was as shocked that someone knew who he was as I was for meeting him. He paid with a NASA federal credit union VISA, and that's how I knew who he was. Ironically, the VISA had a picture of him on it saluting the flat on the moon.
Nicest was Peter Murphy (Bauhaus ) I am a big fan of his music and he was both kind and gracious. Worst Patrice Pike (Sister 7) I am a professional sound guy, and I expect at least a level of professionalism from the artist.
The nicest that I've met was John Darnielle, the lead singer of The Mountain Goats.
He was incredibly polite, he signed people's stuff for free after the show, and he would talk with people for a minute or two. He asked me about my shirt, which just said that word Glastonbury on it with a sword down the middle. He asked me about the music festival, but I him that I actually got it from one of the museums in Glastonbury.
I've met several at conventions, and none of them were blatantly rude. The only one I was somewhat annoyed at was Allison Mack (Chloe from Smallville) because she signed her autograph with a dark marker on a photo of her wearing a denim jacket, and the signature is very hard to even make out. But I doubt she even noticed.
I met Nicholas Brendon from Buffy, and he was nice when I met him but he's constantly getting arrested.