Hard to pick just one, but… Seeing her mouth watering while she’s sucking my cock. Fucking a total stranger that I met a few minutes ago. When she digs her nails into my thigh while I’m fucking her from behind. Possibly the top of the list- When the girl I’m fucking from behind grabs my cock and puts it in her ass without saying a word.
I have a couple of times but it’s more about blind luck and being in the right place than anything. I met one at a gas station in the middle of the night. We flirted a bit, she lived a mile up the road and within minutes I was balls deep in her. It helped that we were both on our way home from some bar and didn’t hook up. After blowing me and finishing me off, she swallowed, looked up and said “I’m Beth by the way”. She was a great FWB for a couple of years. Blind luck.