One common theme is love triangles. Given their complex relationship in the original show, fanfics often explore different outcomes of the love triangle between Addison, Derek, and other characters. Another theme is redemption. Addison might be written as trying to redeem herself in Derek's eyes or in the eyes of others.
Well, it's hard to say who the 'best' is as it depends on personal taste. But some popular authors in the fanfiction community might be those who really capture the essence of Addison and Derek's complex relationship. For example, some authors who are good at writing detailed emotional scenes between them.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfics. Just type in 'Addison Montgomery Derek Shepherd' in the search bar, and you'll get a plethora of stories. Another option is Fanfiction.net, though the quality may vary more there.
One interesting plotline could be them getting a second chance at love. Maybe after years apart, they meet in a different city and rediscover their feelings for each other. Another could be a plot where they have to work together on a very difficult medical case that brings them closer.
One common theme is regret. Fans often explore how Addison and Derek might feel about the way their relationship ended in the original story. Another theme is second chances. Writers like to imagine if they could start over and get it right this time. Also, the theme of love that endures through difficulties is popular, as their relationship was quite complex and had many challenges.
One common theme is her struggle to balance work and pregnancy. Addison has a demanding job, and in fanfics, this often creates tension as she tries to take care of herself and the baby.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created stories, and you're likely to find a good number of Addison Derek fanfiction there.
There could be a fanfiction where Addison and Derek work together on a complex medical case. In this story, their professional respect for each other blossoms into a rekindled romantic interest. As they collaborate, they start to remember why they were attracted to each other in the first place, and the story unfolds with their personal and professional lives intertwined.
There might be a plotline where Addison's daughter faces some sort of family crisis. It could be related to Addison's past coming back to haunt them, like an old rival of Addison's causing trouble for the family. This would force her daughter to be strong and stand up for her family, while also learning more about her mother's history.