There are a few girls that I definitely will remember for a long time but I know I'm never going to forget the first girl that I had a serious relationship with. I made a post here about this recently.
I was 21, at that point a late bloomer and had only had sex with 1 girl prior to her. I started dating her (a 19 year old art school student) who I fell in love with (or was deeply infatuated with) very quickly. We had sex on our 3rd date (almost our 2nd, stayed the night and messed around but didn't actually fvck). Anyway, many of the experiences with her have been deeply etched in my memories and I still think about her 10 years later. She wasn't "hot" (face maybe an 8 but not exactly a great body) but I found her very attractive in her own unique way. What really got me was how much we connected and how much fun we had together, whether it was staying home smoking weed and playing video games watching youtube, travelling to fun places, or going to parties. We shared many similar interests and sense of humor and to this day I've met very few women who I have connected with like that. Looking back on it objectively though, the relationship was riddled with numerous problems from early on and was kind of a disaster, and I think the mutual toxicity of it prolonged what should have otherwise been an intense, 1-3 month FWB/fling. I don't think back on it with regret or "what if" scenarios, nor do I idealize her or the relationship, but man, to deny that it had an impact on me is just foolish. It completely changed the course of my life. The first time we kissed it literally felt like I "came online" for the first time, as if I was an unconscious zombie plugged into the matrix before meeting her. The "highs" of the relationship were so strong that I still felt them a year after breaking up, and it altered the course of my life in a radical way.