One popular 'Marry Britannia OC fanfic' might be 'A New Britannian's Journey'. It focuses on an original character's experiences in the Britannian world, with a lot of character development and interesting plot twists. Another could be 'OC in the Heart of Britannia' which delves deep into the politics and social structure of Britannia through the eyes of the OC. There are also many others out there, you can often find them on dedicated fanfic websites.
I'm not entirely sure which 'Marry Britannia OC fanfic' is the absolute best as it varies from person to person. But some fics gain popularity due to their unique take on the world of Britannia. Some might explore the cultural clashes between the OC and the Britannian society in great detail. They might show how the OC tries to adapt to the different values, traditions and power structures in Britannia. Others may focus on the OC's role in changing the fate of Britannia, either for better or for worse. You really have to explore different fanfic platforms to find the ones that suit your taste.
Well, it depends on what you're looking for in a 'Marry Britannia OC fanfic'. If you like romance - centered ones, there are some that pair the OC with a well - known Britannian character. For example, there's a fic where an OC, a commoner from a conquered area, falls in love with a Britannian noble. The story then unfolds as they try to overcome the social barriers between them. However, if you prefer action - packed stories, there are fics where the OC gets involved in the military or rebel activities within Britannia.