It's a toss up for me, so I'll list a couple...
DD was 3 last trip, and spent the entire week in princess gowns. She was a very tiny, very outgoing little blonde princess, who totally lit up the moment she saw just about any character. (And I was really shocked at how many characters responded to her. Seriously, it was insane how much attention she got.)
We were walking through Fantasyland trying to find my DM and DS, who had opted not to wait in line to meet Rapunzel and Cinderella. All of a sudden, Lady Tremaine and the two step sisters walk by on their way to their meet and greet. DD yelled, "Hi, I love you!" at them, and Anastasia turned around, and saw DD, who raced over to her and gave her a hug. Anastasia then kidnapped my daughter. She took her by the hand and marched after Lady Tremaine and Drizella, who had started their meet and greet. They then proceeded to finish with the girl they were posing with, then play with DD (and us) for about 10 minutes while the line built up and up. They were hilariously nasty to anyone who tried to cut them off or race in for a photo (being villains, I suppose they are allowed to do that.) They informed DD that Cinderella was a liar, and they were really very nice, and Cinderella just tells mean stories about them. When Lady Tremaine found out we had "lost" DS, she asked for tips on how to lose a child since she had one or two she wouldn't mind losing. It was a hoot, and completely unexpected. We hadn't been looking for them, we hadn't known they would be around, and it became one of the high points of the trip.
DS doesn't care that much about characters and never has, but he does like Star Wars, so we booked both Star Wars meals last year. DD doesn't really know who the Star Wars characters are; she was in her Sofia the First gown, and we thought she wouldn't have much interest in the villain meet and greet meal, and might even be scared. Imagine everyone's surprise (including his!) when she raced up to Greedo and gave him a huge hug and kiss, and then ran back to the table to give him all the presents she had traded from the Jawas because "he was her favorite and she loved him so much!" Greedo started pantomiming that he loved her, and came back and played with her again later. We decided that we are not showing her Episode IV any time again soon. (Also, that evening, she decided she was in love with Darth Goofy, and they danced together for a while. What is it about our girl and Bad Boys?)