That's a debate, but I would say it is ultimately Sailormoon.
So here's the line up:
Sailorpluto: Sailorpluto is arguably the one with one of the more powerful abilities.  She can stop time for something like 4 or 5 minutes, but the drawback is that it will kill her if she does.  So yeah, she's powerful, but the one thing that makes her so formidable would give her enemy an automatic win if she was by herself because her opponent would only have to wait 4 or 5 minutes before he/she/it would be unfrozen.  Pluto would only give someone an upper hand if someone else was there to finish off the baddie while she died.
Sailoraturn:  Saturn is supposed to be one of the ultimate senshi...in the solar system, not the universe.  She has the ability to destroy a planet, which is her big bad power.  However, Galaxia has been shown to be able to destroy planets and entire civilizations across the galaxy, so...and Saturn has the conscience to keep her back.
Galaxia:  Galaxia is purportedly the most formidable senshi in the galaxy and was given the title by Kakyuu "Senshi of Death" (which was Saturn's title until she became "good" and became the "Senshi of Silence").  She was shown in the manga to destroy entire planets with the flick of a finger and she has no morality when she's bad.  She can kill the other senshi easily and her Sailor Animamates who are weaker than her can kill the inner senshi easily, too.
Moon:  However, as we know, Sailormoon beats her.  It takes a lot, but Usagi's Silver Crystal is supposed to be the crystal that grants life, while Galaxia's Sapphire Crystal is supposed to destroy life.  Because Usagi has that crystal, Galaxia wants to kill her so she can use both to make her own Shadow Galactica domain.  While Sailormoon's abilities can destroy the Sailor AnimaMates (Starlight Honeymoon Therapy Kiss), it can't stop the AnimaMate Inner and Outer Senshi (AnimaMate Venus deflects it easily, calling it "a soft breeze").  However, Sailormoon's Silver Moon Crystal Power Kiss can and does kill the AnimaMate Inners and Outers.  When she fights Galaxia, however, it seems that they're evenly matched.  The combined planetary attack by the Outer and Inner AnimaMates destroys her Eternal Tier (only for her to recreate it with the Silver Crystal) and in the anime, Galaxia is able to cut it in half with her sword, effectively destroying it.
But, in the end, Usagi's selflessness and love are her apparent strongest weapons as they allow her to utilize the Sailor Crystals.  In the anime, this purifies Galaxia and dissolves Chaos into the universe where it should be.  In the manga, the ability to use all of the Sailor Crystals is called Lambda Power and is used to destroy Chaos and reset the Star Cauldron.  Being able to use Lambda Power defines "Sailor Cosmos."  So technically Sailor Cosmos is the most powerful senshi, and she's what Sailormoon can become.