I was nearly killed by Emma Caulfield (Buffy, 90210, etc) at a friend's barbecue, so that's a memorable celebrity encounter that stands out to me. She was in charge of grilling the chicken, but when my friend and I got our pieces, they were almost raw. We didn't eat it, of course, but we always joke that the chick from Buffy tried to poison us. And on top of that, she was....well, I would describe her as charm-free. But maybe she was just having a bad day.
My mom and I drove from KY to MA to see Maura Tierney in a play there about ten years ago (I've met her a few times and she's lovely) and while we were exploring the town, Casey Affleck and Summer Phoenix approached us and asked for directions. They were very nice, but I wanted to say, "You're from MA and we're from KY, we're even more lost than you are!" lol
I spotted Rosie O'Donnell with one of her sons at LaGuardia once and essentially pointed in her face like a crazy person because I was so surprised and excited to walk right past her. She said hello and kept moving, but I felt like I'd been rude, so I sat down at my gate and tweeted her an apology. She almost immediately tweeted back that it was no problem, and started following me on Twitter. I just thought that was nice of her.
When Kelli Giddish (Rollins on SVU) got her start on All My Children, I became a big fan and started a little fan message board dedicated to her. Her dad found it and sent me a message, and he and I became friendly. So just before she left AMC, I was going to be in NYC for a few days and I asked her dad if she might be willing to meet up with me - and she was! Believe it or not, she actually came to the hotel my friend and I were staying at and picked us up, and then bought us breakfast and spent the morning with us in Bryant Park. She went so far beyond what 99% of celebs would do (even if they aren't hugely famous), it will always blow my mind that she was that kind and open. Ironically, she was in the middle of filming a tiny guest spot on L&O:CI at the time, so I guess that's when the L&O folks first became aware of her.
I've also had the privilege of meeting Boy George (the love of my life since I was five years old lol) at a few meet & greets and he has been an absolute dream come true each time. The last time, it had been almost two years since I'd seen him, but he was like, "I know you!" and gave me a big hug. Plus he recognized my mom, gave her a big hug and told the photographer that he was going to take extra photos with us, even though we'd been told by the staff that it was forbidden. I could have been hit by a bus two seconds later and would have died the happiest person on earth.
But the nicest famous person I've ever met, hands down and without question, is Lucy Liu. I wish everyone could meet her someday, not even because she's famous, but just because she's such an extraordinary human being who is the epitome of kindness and generosity.