One great 'enemies to lovers' fantasy novel is 'A Court of Thorns and Roses' by Sarah J. Maas. It has a great build - up of the relationship between Feyre and Rhysand. Another is 'The Cruel Prince' by Holly Black. The dynamic between Jude and Cardan is full of tension and eventual love. Also, 'Shadow and Bone' by Leigh Bardugo has some elements of this trope as well.
Some excellent 'enemies to lovers' fictions are 'Cruel Prince' by Holly Black. Jude and Cardan are enemies in the faerie world. Jude is a mortal living among the faeries and Cardan is a cruel prince who bullies her. But over time, their relationship changes into something more passionate. 'The Cruel Prince' shows how power dynamics and initial hatred can transform into love. Also, in 'The Vampire Diaries' book series (although it has a TV adaptation too), Elena and Damon start off as enemies. Damon is a vampire who seems menacing to Elena at first, but as the story unfolds, they develop a deep and complex relationship that has elements of love despite their initial enmity.
One possibility for the 'Best of Enemies' back story is that they had a shared past with a third party. This third person might have pitted them against each other, either intentionally or unintentionally. Say this person favored one over the other, and the unfavored one developed resentment towards the favored one, starting the enmity.
The 'Best of Enemies' back story might involve a complex relationship between two characters. It could be that they started as rivals in a competitive field, like business or sports. For example, they might have been competing for the same promotion or championship. Their rivalry could have been fueled by jealousy, different ideologies, or personal pride.
To a certain extent. It takes elements from real-life situations and weaves them into the narrative, but there might be some fictional enhancements for dramatic effect.