I am a sucker for a good friends-to-lovers story. Even better, unrequited love/one friend has secretly crushed on the other for years. There can be pitfalls to this trope if not done well (especially if one friend ends up being a huge oblivious jerk when he or she doesn’t realize the other has romantic feelings), but when this trope is done well, I find it so incredibly romantic and satisfying. I think one reason why I like this trope so well is also because by default, since they are friends, the hero and heroine know each other before the novel starts or become friends over the course of the novel, and therefore there’s no insta-lust. A lot of good slow burn romances are friends-to-lovers and those are my preference.
I also love enemies-to-lovers, especially when the author is good at quick, witty His Girl Friday-type dialogue.