Sailor Moon features some incredibly powerful characters. Sailor Moon has blown up shadow universes and Sailor Saturn can bring silence to the entire solar system by dropping her glaive. Sailor Pluto is essentially a demi-goddess, being the daughter of Chronos, the God of Time. Sailor Pluto's mastery over time is severely limited by the rules Queen Serenity placed upon her during her early appointment as the guardian of time. If she broke the rules, would Pluto be the strongest of Sailor Guardian?
Even with those rules, is she potentially among the strongest, though just behind her peers Sailor Moon and Sailor Saturn. Let's analyze Pluto's potential in the series, how the three rules restrict her and whether or not Pluto could really be the strongest there is when freed from her constraints.
The Three Rules of Queen Serenity
The three rules Queen Serenity placed on Sailor Pluto are as follows: Sailor Pluto cannot use the Door of Space and Time to travel through time, cannot abandon her post at the Door and cannot use her powers to stop time. Shortly after these rules are established in the manga, Sailor Pluto breaks all three rules in order to stop Death Phantom and the Black Moon Clan. In the original anime, Sailor Pluto breaks all three rules over the course of Sailor Moon S in order to stop Pharaoh 90's invasion of Earth.
Breaking the rules comes with penalties -- or, at least, the last rule does. When Sailor Pluto freezes time, she cannot do anything while freezing time, other than talk. This time stopping power puts such an intense strain on Pluto's body it kills her -- temporarily. Even with this weakness, this means she can manipulate time, one of the key components of reality. Does that make Pluto, despite her shortcomings, the strongest Sailor Guardian?
Sailor Moon and Sailor Saturn vs. Sailor Pluto
Sailor Saturn is able to eradicate all life in the solar system by dropping her glaive. However, this ability comes with a fatal shortcoming, much like Sailor Pluto's time-freezing ability. Should Sailor Saturn drop the glaive, she'd also die. Both Sailor Guardians have similar world-breaking abilities with deadly drawbacks. However, in both cases, both characters return shortly after using their power-based deaths, though Sailor Saturn is reborn as a young child.
It's ultimately unfair to argue Pluto has more "power" than Saturn since their abilities are difficult to compare. Sailor Saturn cannot freeze time, but Sailor Pluto's destructive output does not compare to hers. While both have incredible might, they are also incomparable.
However, while both Pluto and Saturn are incredibly powerful, their powers pale in comparison to Sailor Moon's abilities. Usagi's power is roughly equal to Saturn's during the Sailor Moon S saga, but her power grows even stronger as the series progresses. In the manga, Usagi obliterates Nehelennia's Mirror Dimension -- an entire universe. Later still is Sailor Cosmos, Usagi's strongest, final incarnation. Her abilities are downright Godlike, with her competing against Chaos, the cosmic force of all evil in the universe.
If Sailor Pluto is the daughter of a god, then Usagi's final form is a god. Sailor Cosmos is able to travel through time without Sailor Pluto's authority, making her able to do everything Sailor Pluto is forbidden from doing without any restraint. There is simply no comparison between the other Sailor Guardians and Sailor Moon herself. While Sailor Pluto is powerful, possibly standing as an equal to Sailor Saturn, Sailor Moon is absurdly powerful.
