Haruka, Michiru, & Setsuna
Although the "Outer Solar System Senshi" name is a misnomer, it is a commonly used term that refers to Sailors Uranus, Neptune, Pluto, and Saturn, as opposed to the Sailor Guardians (Mercury, Mars, Venus, and Jupiter). The four of them - originally and most of the time just three, are similar in that their civilian forms are all poorly developed but still Really Cool, which is obviously all they need to be some of the most popular characters in Sailor Moon fandom.
Haruka, Michiru, and Setsuna are less than receptive of Hotaru. Of course, they're all considerably older than her so they don't interact with her very much, even when they all attend Infinity Academy. However, the three of them are aware of Hotaru's existence as Sailor Saturn. Their past selves having witnessed her destruction of the Silver Millenium, they are determined to prevent her coming, and so resolve to kill Hotaru, much to the dismay of the Sailor Moon and the Guardians.
In the anime, their determination to kill her is much more pronounced. Haruka, especially, is very outspoken about Hotaru's imminent threat. Even later, after her rebirth, when Pluto brings Hotaru to them upon Nehellenia's arrival, Haruka is shocked and slightly outraged at Hotaru's presence. In the manga, however, it is shown that the three of them seem to pity her. They see that her body is part machine, but don't openly show any revulsion to her.
Their attitudes change very quickly. Saturn's arrival and destruction are necessary to cleanse the Earth and Sailor Moon will restore it. With Pluto's assistance, she sacrifices herself to save the planet. They are shocked and realize that she is, indeed, one of their own, though her powers are far more destructive than their own.
The four of them eventually become a family. In the manga, Hotaru's father is killed in the struggle against Pharaoh 90. After her rebirth, they take her with open arms and make a promise to raise her, realizing that they were mistaken about her, and taking care of Hotaru takes priority over their watch over Sailor Moon. In their little family, they live a happy life: Setsuna takes care of her discipline and education, Michiru takes care of her feedings and general health, and Haruka of her diapers and of being a playmate. Fun times for all!
The four of them are a great team, nearly inseparable - the most obvious dividing line is between Haruka & Michiru and the others. Otherwise, though, they develop a great family dynamic through the series.