My Favorite Romantic Relationships in Book Series!

I have come to a realization recently about what kind of relationships I enjoy in books and what feels memorable to me. I do love romance novels, but since they are usually one and done, I don't feel like I really have enough time to grow attached to the characters or the relationship. This means that they just aren't that memorable to me. I also love slow burns, and in a single book, I feel like there just isn't enough time to develop it and still have it be paced well. I also love situations that aren't necessarily romantic, but can show a strengthening bond between characters. This leaves me with my conclusion: I like romantic relationships from series that are not primarily romance.