There's a really interesting one where the author gives a different perspective on the fight. It's called 'Naruto and Sasuke vs Momoshiki: An Alternate View'. It focuses more on the strategic aspects of their combat that the anime might not have fully explored.
It's appealing because we get to see more of the characters' thoughts and feelings during the fight. In the original series, there are limitations on how much of their internal monologues can be shown. Fanfictions fill that gap. For example, a fanfiction might show Naruto's doubts and how he overcomes them during the battle with Momoshiki. Also, some fanfictions might explore different relationships between Naruto, Sasuke and other characters in the context of this battle, like how their teamwork affects the fight in ways not shown in the anime or manga.
Another scenario could be that Naruto and Sasuke discover a new power within themselves during the fight against Kaguya. This new power could be related to their friendship or their connection to the Sage of Six Paths. They then use this power to finally defeat Kaguya.
Most likely Naruto and Sasuke. In the original series, they were already very powerful when they faced Kaguya, and in fanfiction, they could be even more so. Their combined strength and determination usually make them the favorites to win.
One exciting moment could be when Naruto and Sasuke combine their powers in a new and unexpected way. For example, they might create a unique jutsu that gives them an edge against Kaguya. It's always thrilling to see how they work together, especially considering their complex relationship.
I really liked 'A New Beginning for Momoshiki in Naruto'. This fanfiction takes an interesting turn where Momoshiki is reincarnated in a different form and has to navigate the world of Naruto from a different perspective. It's full of action and also some great character development for Momoshiki. Also, 'Momoshiki and Naruto: Unlikely Allies' is a great read as it focuses on how their powers could combine in unexpected ways.
There are fanfictions where Momoshiki and Naruto interact in unique ways. Maybe Momoshiki has some knowledge that Naruto needs, and they have to negotiate. Their power levels are compared and contrasted. Naruto's ability to use Sage Mode and his chakra reserves are pitted against Momoshiki's alien - like powers. It creates a dynamic where both characters are forced to adapt to each other's fighting styles.
There could be physical changes too. Naruto might have a new form that combines his own features with Momoshiki's. His body could become more resilient, like Momoshiki's, and he might have a different chakra signature. This new form could attract the attention of other villains in the Naruto universe, either out of fear or the desire to steal his power. Moreover, his relationships with other characters like Sasuke could be affected, as they have to adapt to this new Naruto who has absorbed Momoshiki.
For the ninja world in the fanfiction, it might bring a sense of peace as Momoshiki was a powerful threat. However, it could also lead to new enemies emerging, seeking revenge for Momoshiki's death. Naruto might also have to deal with the political fallout, as some villages might see his act as an overstep of power or a sign of aggression.
A Momoshiki fanfiction could focus on his relationship with Boruto in an alternate universe. For example, they could end up as allies instead of enemies. They might team up to face a greater threat that endangers both their worlds. This would involve a lot of character development for both of them as they learn to trust each other.