I had two good interactions with girls. Full disclosure: this was in Tennessee, and girls generally respond much better to me in the South.
One was a bartender. I asked about her ethnic background because she was a dead ringer for Betelgeuse (note: she was in fact half-slav). Later my parents said she really liked me, but I completely missed the signals.
Another was in a stairwell with a girl who must have been a runway model, because she was reed-thin and walked like a princess. I asked her “You have an elegant way about you, did you learn that by practicing or what?” She said her grandma was like that and it just seemed natural to her. She then introduced herself and asked if I lived on that floor. I said I was just visiting and would be gone in a couple days. Realized later she was trying to say she wanted to see me again and I should have asked for her number. Normally I’d discriminate against city girls but she seemed very classy and sexy. Not to mention foreign, so maybe she would actually be classy.
The lesson I generalized from these (and past interactions) is my best pickup lines are from asking a pure curiosity question based on an incisive observation about the girl. Sometimes it flops, but the success rate is pretty good.