It depends on personal preferences. Some might like the anime for its visual and audio elements, while others might prefer the manga for its original storytelling and detailed art.
One great 'Usagi Drop' pregnancy - related fanfic might be the one that focuses on how the characters adjust to the new addition in their family. It could explore the emotional journey of the main characters as they prepare for the baby.
I'm not sure which ones are the most popular specifically, but you can start by looking on fanfic websites. Check the tags related to 'Usagi Drop' and 'pregnancy'. There might be some fanfics with high ratings or a large number of reviews that could be great.