Weirdly enough, I don't go for the 'love at first sight' angle when the masks are off; the fics that stick with me are the ones where they fight first. Not physically, but that initial clash where he's a showboating flirt and she's defensive and sharp, their whole dynamic is built on mutual annoyance that's secretly fascination. That friction is the good stuff. The best trope, for my money, is the 'accidental vulnerability' moment—maybe Chat catches Marinette crying over something unrelated and his entire persona drops, or she sees him exhausted after a fight and offers him a croissant without even thinking. It's not grand; it's small, human, and it cracks the shell.
I also have a soft spot for the identity shenanigans, obviously, but not the big reveal. I mean the little almost-moments that drive you crazy, where one of them says something suspiciously specific or uses a turn of phrase only the other persona would know, and they both just brush it off. The tension of them being so close to figuring it out, yet so stubbornly blind, is a special kind of torture that I absolutely love reading.