20Ron & Hermione (the early years)
Readers who accept "19 Years Later" as canon, (we're aware that some don't) know that Ron and Hermione eventually get married and have a couple of kids. They get along so well that the couple uses a blended name. The Granger-Weasley family gets on famously now.
But that was very much not the case when they were students.
Before Hermione was Ron's friend, he called her a "nightmare" and opined that her habit of correcting people is why she "hasn't got any friends." Jumping ahead to 4th year, it doesn't seem to occur to Ron that Hermione is a girl, let alone a viable date to the Yule Ball—until he's totally desperate. By that time, Hermione had been asked at least twice. Then Ron had the nerve to tell Harry that Hermione must have been lying about having a date at all. At the same time, Hermione's jealousy when Ron was dating Lavender Brown was the stuff of legend. You can't claim the moral high ground after you send a flock of birds at someone's head. Dating Cormac to make Ron envious? Ew! We'll forgive Ron for his disappearance during Harry Potter and the Deathly Hallows, mostly because the ball of light in his heart entranced us.