"You're cute!"
But really, I remember more what people have done than said.
When I turned 21 and was about to spend my birthday alone with only my mom, my best friend called and demanded we do something that night. It was late, I said no, but she insisted. She had gotten on the train and come down into my neighborhood as a surprise.
In high school, I worked with this guy I liked in the summer. When fall rolled around, I saw him on the train with a group of his friends. I waved awkwardly at him and he abruptly left his group, swung around the poles of the subway car and stopped right in front of me. We basically just asked each other how we were (and he wasn't interested in me) but I'll always remember he did that.
(It think it was a big deal for me, because I used to have a childhood friend in elementary school that I was close with in our afterschool program, but who shunned me in our day classes. I was pretty much an outcast in my day classes and the saddest thing was, I accepted it and she didn't do anything to change it. This time, my friendship wasn't dependant on the venue or the social atmosphere. He really cared and didn't care who saw.)