Goku was the first Super Saiyan in Dragon Ball Z, but the legend of the Super Saiyan was quite old, as shown by Vegeta himself being very particular about the fact that a “legendary Super Saiyan” even existed. Bardock and Broly are also called the legendary SSJ, but who is the real one anyway?
In order to understand who the legendary Super Saiyan is , it is first necessary to know that this title has different meanings.
The first is precisely about the one who gave rise to the legend of the Super Saiyan, the first Super Saiyan, which unfortunately is a subject that the canonical world of Dragon Ball has never explored.
This character is revealed to be Yamoshi, an unknown Saiyan who could become a Super Saiyan, and he may have been responsible for spreading the myth down to Vegeta and Frieza’s generation.
Also, in Dragon Ball Heroes, Bardock goes back in time and becomes a Super Saiyan in a fight against Frieza’s ancestor named Chilled.
He was terrified of that and managed to warn his descendants, which in the future reached Frieza, and thus it can be said that (non-canonically) Frieza feared the “legendary Super Saiyan” because of Bardock himself who traveled to the past.
The other meaning of legendary Super Saiyan refers to a very powerful Saiyan who is born every thousand years.
He has power that completely exceeds all other Saiyans and he can access a transformation called the Legendary Super Saiyan. Broly was this rare Saiyan, who had 10,000 fighting power as a baby, who was already as strong as the King of Saiyans.
Therefore, the legendary Super Saiyans are Yamoshi, Broly, and even Bardock.
Goku is not exactly a legendary Super Saiyan in the sense that he is truly legendary or has given rise to legend, but his transformation against Frieza is one of the most iconic in the entire franchise and so it is not surprising that he was called legendary as well.