Pretty guardians who attend school by day and find evil by moonlight is the premise behind the Sailor Moon franchise. That plot has endured through its manga, original anime, several movies, a live action adaptation, reprints of the manga, and a remake of the anime. At the core of the Sailor Moon franchise, however, is Usagi Tsukino's belief that love conquers all. With that kind of belief, the relationships in her life are very important to her.
Usagi is the Moon Princess, but she's also Sailor Moon. The guardians who would protect the princess in their past lives become her cohorts in the present. They also become some of her closest friends. Usagi is definitely closer to some of her Sailor Guardians than others, however.
10 Sailor Saturn
It would be impossible to rank Sailor Saturn higher than the last spot. She is, after all, the Sailor Guardian that is most feared before her actual appearance. Her civilian guise as Hotaru isn't exactly close to Usagi either.
Instead, Hotaru is close to Chibiusa, which makes her more like a young child Usagi is friendly with through a baby-sitting venture than a close personal friend. It doesn't help that Hotaru doesn't get a lot of time to develop in any version of her story before she uses her power and is reborn as an infant all over again.
9 Sailor Pluto
Slightly higher than Sailor Saturn, Sailor Pluto primarily earns this spot because she's more of an employee than friend to Usagi - especially in the glimpse fans get of the future. There's respect between them, but not friendship.
Setsuna is the one person the future Queen Serenity trusts to guard the space-time door. She also trusts Setsuna to look after her daughter, which is some pretty high praise in a future where the Black Moon Clan still want to take over. Setsuna and Usagi, however, don't do a whole lot of interacting outside of teaming up on the battlefield.
8 Sailor Neptune
Before Sailor Moon and Sailor Neptune end up on the same side in battle, Michiru and Usagi cross paths in real life. Michiru doesn't exactly endear herself to Usagi when she spends her spare time flirting with Mamoru to get a read on him.
Much like Setsuna, the two don't spend a lot of time together outside of the battlefield. The difference is that, at one point, Usagi does attempt to befriend her. The two are more casual acquaintances, though they do appear to genuinely like one another once they've battled a few bad guys together.
7 The Sailor Starlights
Initially, there's plenty of conflict between this trio of Sailor Guardians and Sailor Moon's team. After all, the Sailor Starlights appear to be interfering with Sailor Moon's team going about their normal duty of protecting the planet. The Outer Guardians in particular are harsh with them. In defense of the Sailor Starlights, they were just looking for their own princess.
As is par for the course when Usagi meets someone new, she goes out of her way to try to befriend them. That works out for her in some respects, but it also creates a bit of a romantic debacle for her in the anime. The Sailor Starlights first duty is to their princess, which is why they never seem to get particularly close to Usagi, but are more casual friends.
6 Sailor Uranus
Of the Outer Guardians, Sailor Uranus is the one that Usagi becomes closest with. That's initially because she develops a bit of a crush on the new kid. When she discovers that Haruka is Sailor Uranus, it doesn't lessen her interest.
Instead, it only causes Usagi to repeatedly appeal to them that they need to unite their forces. Eventually, Haruka and her friends agree. Their connection with Usagi is what gets them to stand with the Inner Guardians, something they try to avoid doing when they first appear.
5 Sailor Chibi-Moon
Sailor Chibi-Moon outranks the Outer Guardians because terming Chibiusa and Usagi friends is a bit of a stretch. They develop a deep love for one another, but it's one born out of familial connection.
Initially, Chibiusa isn't a part of the fight because her Sailor Guardians abilities haven't been accessed. Once she becomes Sailor Chibi-Moon, however, she tries hard to stay by Sailor Moon's side. She has a hard time reconciling the teenage girl who doesn't know what she's doing with her poised and powerful mother, which is the big source of their tension a lot of the time.
4 Sailor Mars
Though Sailor Mars is the second Sailor Guardian that Usagi helps to awaken, the two aren't the closest of friends. In the anime, Rei and Usagi rarely see eye-to-eye on anything. In the manga, Rei is a bit more distant from the other Sailor Guardians as a result of her being a shrine maiden.
Rei spends just as much time away from them fulfilling her shrine maiden duties as she does with them fulfilling her Sailor Guardian duties. She takes her role seriously, but that can also cause her to be more serious with Usagi and the others, not allowing her to treat them all as her closest friends, even though they clearly are.
3 Sailor Venus
Minako and Usagi could have been sisters. Their similar appearance allows Minako to pass as the Moon Princess as a decoy. As the true leader of the Sailor Guardians, Sailor Venus tends to be the most experienced in the field, and the one most willing to sacrifice herself for her princess. That's all a part of her duty.
In her civilian life, she becomes close with Usagi very quickly. Despite Usagi being the one with the leadership role now as Sailor Moon, there's never tension between them. They even share a lot of the same interests, making them just as likely to hang out off the battlefield as on.
2 Sailor Jupiter
Makoto is the gentle giant of the group. Her large stature and superior strength can make her seem intimidating to people who don't know her, but Usagi takes a liking to her right away, befriending her as soon as she joins the same school.
When Makoto awakens as Sailor Jupiter, Usagi is thrilled to have yet another one of her new friends turn out to be a Sailor Guardian. Even when Sailor Jupiter is better at activities than Sailor Moon, she never makes her friend feel inferior. Always supportive, and always ready to lend a hand, she becomes one of the closest Sailor Guardians to her.
1 Sailor Mercury
Ami Mizuno is Usagi Tsukino's most loyal friend. Usagi is the first person to see Ami as more than just her superior brain. Ami sees beyond Usagi's lack of academic skills as well. Though the two couldn't be more different, they balance one another out.
While Sailor Mercury is quiet, a thinker, Sailor Moon is loud and all about taking action (or running away in her early tenure). The two support one another through their weaknesses. Sailor Mercury is able to boost Sailor Moon in battle just as Ami is able to help Usagi in school. Sailor Moon brings Sailor Mercury's plans to life in the same way that Usagi brings Ami into teenage social situations. They're the best of friends.