In the world of Avatar: The Last Airbender and The Legend of Korra, the Avatar is the most powerful and influential person that exists. As the master of all four elements, the Avatar functions as a bridge between the spiritual and physical worlds, bringing balance and harmony to the four nations.
It's a role and responsibility not to be taken lightly. Some even chose to run from it. Others take their time to truly come into their own as an Avatar, though in exchange, their ability to perform their duties suffer massively. Similarly, the Sailor Senshi of Sailor Moon act as guardians of the galaxy and know full well the responsibilities and expectations of keeping balance. Were they to step into the world of ATLA and TLOK, how would they fare?
10 GREAT: Rei Hino/Sailor Mars
Having worked as a Shrine Priestess, Mars already has the spiritual aspect of being the Avatar down. She can sense evil energy and spirits, utilizing Ofudas - charms or wards in Japenese culture said to bear good luck - to cleanse them. Additionally, Mars often shows a knack for leadership, often filling in for Sailor Venus in leading or rallying the Inner Senshi.
9 NOT SO GREAT: Ami Mizuno/Sailor Mercury
Mercury is often overlooked when it comes to pure firepower, but she's far from weak. As the Milky Way's smartest Senshi, Mercury's tactics and thinking often proved vital in overcoming threats.
Her greatest strength is, however, also her greatest weakness. The Avatar sometimes has to make decisions in a moment's notice. Mercury would probably try her best to analyze a situation before making a choice, which cannot always be done in the Avatar Universe where tensions have a tendency to run especially high.
8 GREAT: Michiru Kaioh/Sailor Neptune
Like Mars, Neptune has a natural affinity for precognition, though the Outer Senshi's abilities are far more powerful. Her powers allow her to more clearly see through illusions and even peer into a person's past life, which would undoubtedly help with contacting the other Avatars.
Additionally, Neptune takes her duties very seriously, going so far as not to let anyone help her because she feels that her mission and mistakes are her own to fulfill and fix. Though many Avatars had teams helping them in their goals, an Avatar's duty can only ever be fully completed by them.
7 NOT SO GREAT: Minako Aino/Sailor Venus
Being the leader of the Inner Senshi puts Venus in a pretty good spot when it comes to Avatar qualifications. A competent leader, Venus understands her duty and is able to balance it with her interests and dreams.
Unfortunately, Venus would struggle to understand and connect with the spiritual aspect of The Avatar. She's never given an inclination about her personal spirituality, and her attachment to the material world would probably contribute to her problem. Though it's something she can eventually overcome, her functionality as an Avatar would be compromised until that point.
6 GREAT: Hotaru Tomoe/Sailor Saturn
Saturn fully understands the weight of wielding immense power, capable of destroying entire galaxies and even holding power over evolution. She is fully prepared to make sacrifices as well, having demonstrated the resolve to end her own life if it meant defeating evil (since using the full extent of her powers causes her own death as well).
5 NOT SO GREAT: Usagi Tsukino/Sailor Moon
Usagi is by no means a lousy leader, princess, or Senshi, but she would have difficulty accepting the full weight of her new responsibility. Though the Avatar is meant to live among humans to become more human themself, Usagi's attachments would probably get in the way of her duty. This particular dilemma is common for Avatars, most notably Aang.
4 GREAT: Setsuna Meiou/Sailor Pluto
Being the guardian of space and time, Pluto already has a lot of responsibility on her shoulders. She's steadfast in her duty and knows the consequences of neglecting it. As a result, Pluto would be able to make decisions with conviction and honesty, which is vital in fulfilling the role of the Avatar.
Being the Avatar would allow Pluto to live as she's always wanted to live, actively participating in life instead of being confined to a single spot.
3 NOT SO GREAT: Seiya Kou/Sailor Star Fighter
While Fighter is fully dedicated to her duty, she is easily distracted by more worldly pursuits, namely Usagi Tsukino. Fighter often puts her mission aside to spend time with Usagi, and even becomes conflicted about her actions when the other Starlights request she cut off all contact with Usagi.
The balance of the world can't wait for her to stop trying to get the Moon Princess to fall for her.
2 GREAT: Makoto Kino/Sailor Jupiter
On the surface, Jupiter may not look like she has what it takes to succeed as an Avatar, but her sense of responsibility speaks for itself. An orphan, Jupiter has to not only fulfill her duty as a Senshi but also balance that with the weight of financially supporting herself while in school. She never seems overwhelmed by her situation and could easily handle any extra duties that came her way.
While noted for being a fighter, Jupiter only does so in defense of those who can't defend themselves, which is a massive part of being the Avatar.
1 NOT SO GREAT: Sailor Galaxia
As the strongest Sailor Senshi in the galaxy, Galaxia could wield the powers of the Avatar with little effort. She's also extremely devoted to her duty, and wouldn't have any problem making decisions some Avatars may spend considerable time questioning.
However, it's that same strength that would be Galaxia's downfall. She was willing to sacrifice everything in the galaxy if it meant she'd win, and balance cannot be restored to a world that no longer exists.