I'm not sure specifically which ones are the most popular. However, you can try searching on fanfic platforms like FanFiction.net or Archive of Our Own. Usually, the ones with high ratings and a lot of reviews might be popular among fans.
Fanfiction should be inclusive and appropriate. When it involves elements like 'futa' which have adult connotations, it is not acceptable for general audiences. The 'Ben 10' franchise has a large fan base of different ages, and fanfiction should be made in a way that respects all fans. So, this kind of fanfiction is not suitable for all.
One popular 'ben 10 futa fanfic' story could be about a new alien form that Ben discovers which has unique powers and how he uses it to save the day. Another might focus on the relationships between the characters in a more in - depth way with the futa aspect adding an interesting twist to the normal dynamics.
I'm not sure specifically as 'lemon' often implies inappropriate or adult - themed content which I don't promote. However, if we consider general Ben 10 fanfiction, there are many that explore different adventures of Ben and his alien forms in various new universes or against new enemies.
You can look for them on major fanfic websites such as FanFiction.net or Archive of Our Own. These platforms have a large collection of fan - created works.
One possible plot could be that Ben 10, with his alien - transforming powers, teams up with Futa Charmcaster in a dimension - hopping adventure. They might face a common enemy that is threatening multiple universes. Ben's various alien forms could provide different skills to combat the threats, while Charmcaster's magic could be used for both offense and defense, and their combined efforts lead to the salvation of the universes.
Obviously, Ben 10 is one of the main characters. He has a wide range of powers thanks to the Omnitrix. And Futa Charmcaster is also a key character. She is a powerful magic - user. Their interactions and how they combine their different powers are likely to be central to the fanfiction. Ben's courage and Charmcaster's mysterious magic make them an interesting pair to follow in the story.
To write a great 'ben 10 futa fanfic', you need to focus on character relationships. For example, how would Ben's friends react to his new futa - form? Also, create a compelling plot. Maybe there's a threat that only Ben in his futa - form can defeat. And don't forget about good writing basics like grammar and pacing.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a wide range of fanfiction, and you can filter the search to exclude any inappropriate content. Just search for 'Ben 10' and look through the results to find the ones that suit your taste.