In the novel's ending, the mystery is finally resolved. The identity of 'A' is revealed, and the girls are able to put an end to the constant torment they've been through. They also learn a great deal about themselves and their friendships along the way.
Spencer is also very popular. Her intelligence and competitiveness can be the center of many fanfictions. Writers can write about her academic struggles, her relationship with her family, and how she uses her smarts to try and figure out the identity of 'A'.
One idea could be exploring a different backstory for one of the main characters. For example, what if Aria had a secret relationship with someone before she moved back to Rosewood. This could add a whole new layer of mystery and drama to the story.
In the end of the Pretty Little Liar novel, the girls triumph over 'A'. The revelation of 'A's identity gives them a sense of relief and they can then focus on rebuilding their lives and mending their friendships.
Another reason is that they both have had their own share of troubles in the show. By pairing them in fanfiction, it gives the writers a chance to show how they can heal each other. Spencer, with her family problems, and Caleb, with his past in foster care, can find solace and love in each other. This aspect of two damaged souls coming together is very appealing to many fans who write and read fanfiction about them.
One common theme is forbidden love. Since there are so many secrets and complications in the show, this often translates into their relationship in fanfiction. Another theme is mystery, as both characters have their own secrets, and fanfic writers like to explore how these secrets affect their relationship.
In fanfiction, Wren and Spencer often have a complex relationship. Sometimes, they are depicted as having a secret, passionate affair. Wren's mysterious nature adds an element of intrigue to their connection, while Spencer's intelligence makes their interactions intellectually stimulating.
I'm not sure who the 'best' is as it's subjective, but some popular authors might be those who have a large following on fanfiction platforms. Look for ones with high ratings and lots of reviews. For example, there could be an author named 'RoseWriter' who is known for her detailed characterizations of Emily and Spencer.
One key element is the character interaction. How Spencer, with her intelligence and complex personality, interacts with Dean, who likely has his own set of skills and traits. Another is the setting integration. Incorporating the dark and mysterious world of Pretty Little Liars with Dean's possible role in it. Maybe he could be helping to uncover the 'A' mystery or dealing with the other secrets in Rosewood.
There are many great writers out there. Some popular ones are often those who are really good at capturing the personalities of Spencer and Toby as they were in the show. For example, some writers on fanfiction.net have a large following for their PLL fanfics. But it's really subjective, as different people like different writing styles.