A really cool fanfiction could be set in an alternate universe where Crabbe and Goyle are actually very intelligent but pretend to be dim - witted. They could be working on a secret project that will change the wizarding world, and Harry Potter unknowingly stumbles upon their plan.
One interesting fanfiction could be about Crabbe and Goyle having a change of heart and turning against the Dark Arts. Maybe they realize the harm Voldemort is causing and decide to help Harry secretly. Another could focus on their friendship before they got involved with the wrong crowd at Hogwarts, showing a more innocent side of them.
I'm not sure about the absolute best, but there are some authors who have a good reputation. Some of the ones who are often recommended might be those who really dig deep into the characters of Crabbe and Goyle. They don't just write them as the typical bullies but give them more complex personalities.
One interesting Goyle fanfiction could be one where he has a change of heart and tries to break away from his family's dark views. It might explore his internal struggle between loyalty to his pure - blood family and his own sense of right and wrong.
It might be used to show different physical capabilities. For example, Crabbe could be slower in running due to his weight.
Often, the best authors of Goyle fanfiction are the ones who take a relatively minor character like Goyle and give him depth. They might create backstories for him that explain his behavior in the original books. Maybe they show that he's not just a sidekick to Malfoy but has his own hopes, dreams, and fears. These authors usually do a great job of fleshing out his character and making him more than what he seemed in the main series.
I'm not sure exactly how it's described as I haven't read that specific fanfic. But it could be through direct statements like 'Crabbe weighed ten pounds more than Goyle' or more descriptive passages about their physical appearances related to their weight.
One interesting Harry Potter fanfiction could be those that explore alternate universes, like 'The Harry Potter Who Lived'. It shows a different path for Harry where he makes different choices. Another great one might be fanfics that focus on the backstories of minor characters, for example, a story about Neville's parents before they were tortured. There are also fanfictions that pair unexpected characters in relationships, which can be really fascinating to read.
One interesting Harry Potter fanfiction could be a story that explores what would happen if Harry had been sorted into Slytherin instead of Gryffindor. It could show his struggle to fit in with the Slytherin values while still holding onto his own sense of right and wrong.
One interesting Harry Potter fanfiction could be about an alternate universe where Harry is sorted into Slytherin. It shows a different side of his character development as he has to navigate the house known for its cunning. Another great one might be a story that focuses on the Marauders' time at Hogwarts in more detail, exploring their pranks and friendships deeper.
One interesting Harry Potter fanfiction could be a story where Harry discovers a hidden magical ability that was not explored in the original books. For example, the power to communicate with magical creatures on a deeper level, allowing him to gain new allies in the fight against Voldemort.