What small detail can turn a good route into an unforgettable one?

I completely agree! I love a naturally developed romance instead of insta-love tropes where the heroine or usually the LI just falls in immediately. And I love little scenes that demonstrate how MC and the LI are actually emotionally compatible and a good fit for eachother in terms of their values or interests - it feels like wow, these two would actually make a strong long term relationship work and that the relationship is not just built on physical attraction!

like for example an LI is talking about a certain principle or goal he has, maybe he's really ambitious and has big plans for the future, and the MC might respond with how she admires people like that who have a strong drive and how it inspires her to keep working towards her goals too!! Small stuff like that is so cute

I think the most unforgettable route for me is the kind of plot that is full of ups and downs, a bit heart wrenching and tear jerking. The process can make me sad and upset, but I can only accept the happy end of the story. This kind of plot will make me unforgettable for a long time, and I may even remember it and play it repeatedly many times.

Same here !!
In addition, subtle psychological and emotional shifts particularly appeal to me. If a character grows mentally or changes their perspective on things (not necessarily in romance) due to events, even a bad ending feels like a good route to me, if it's well-developed.

I think so too.This kind of description makes me feel like I've gone through a lifetime along with the story.Whenever I look back on all that happened at the end, it feels like things have changed but people haven't.This feeling is truly unforgettable.