Dragon Ball's Rarest Super Saiyan Form Was Only Ever Achieved By One Person
After "Dragon Ball Z" introduces Goku's Super Saiyan transformation, learning more powerful forms becomes key to just about every major character's arc as the franchise progresses. That said, most subsequent Super Saiyan transformations are attainable by multiple characters, like Gotenks going Super Saiyan 3 shortly after Goku in the final "Dragon Ball Z" saga. The rarest Super Saiyan form, then, is one of the few only ever achieved — and likely only ever attainable — by just one character in particular.
The third saga of "Dragon Ball Z" sequel series "Dragon Ball Super" introduces a villain named Zamasu who, in one of two parallel universes, takes the form of Goku and becomes known as Goku Black. His sole Super Saiyan transformation appears similar to the base Super Saiyan form in shape, but turns his hair a shade of light pink instead of gold. Because of its unique color, this transformation is known as Super Saiyan Rosé.
Thus far, even in the "Dragon Ball Super" manga — which has surpassed the anime by a couple of story arcs — Goku Black remains the only character capable of going Super Saiyan Rosé, making it the single rarest Super Saiyan transformation in "Dragon Ball" canon.
Super Saiyan Rosé is practically impossible for any character to achieve other than Goku Black
The definition of Super Saiyan Rosé differs between the "Dragon Ball Super" anime and manga, neither of which is definitively more canonical than the other. In the anime Super Saiyan Rosé is the result of Zamasu activating the first Super Saiyan transformation in his Goku Black form, whereas the manga explains that he's activating his equivalent of the much stronger Super Saiyan Blue transformation.
In either case, the reason he ends up with a unique appearance rather than that of either the typical Super Saiyan form or the Super Saiyan Blue form is because Zamasu was once training to become a Supreme Kai, which is a type of god. By taking on the countenance of a Saiyan when he becomes Goku Black he gains the ability to go Super Saiyan, but his divine energy means that the ability manifests differently than it does for a regular Saiyan without divine energy.
Since no other gods in "Dragon Ball" have or seem likely to ever want to become Saiyans, then, Goku Black should remain the only character to ever go Super Saiyan Rosé, solidifying its place as the rarest Super Saiyan form in all of "Dragon Ball."