As stated in the question, is Goku the first Super Saiyan? If not, who was the first Super Saiyan? Was it:
- Bardock (according to the Episode of Bardock)
- Someone else
Now this question has a definitive answer. This is new information. The first super saiyan was Yamoshi. Akira Toriyama gave an interview recently and he stated the first super saiyan and the first super saiyan god was a saiyan named Yamoshi.
Historically, Probably NO.
Original Super Saiyan was originally thought to be a legend, until Goku first transformed during his fight with Frieza. In a filler scene in episode 66 of DBZ, the first appearance of the Original Super Saiyan was shown, it looks like a Great Ape with yellowish colored fur. Apparently Super Saiyan was not in Great Ape form, unless it's a Golden Great Ape, it was made up by Vegeta's imagination.
Talking about Bardock, he was suspected as the first Super Saiyan, there is no official information to prove he is the first ever Saiyan to transform into a Super Saiyan. At the end of Episode of Bardock, it is revealed that Chilled's knowledge about Super Saiyans was passed to his descendants. It is not confirmed yet that Akira Toriyama wants Bardock to be the first Super Saiyan.
But until now, Bardock was the first Super Saiyan in the series with a known name and appearance.
When Super Sayan was first described in the source material, Vegeta had said that there had once been a super Sayan roughly 1000 years prior to the current timeline.
As far as I know, this Super Sayan from 1000 years ago has yet to be named.
So no, Goku was not the first.
The first super sayian was Bardock father. He was sent back in time from Freza's blast that he sent at planet Vegeta. Where he met Freza's great grandfather (idk how many greats) and that's where the story of super sayians came from. With all his anger at having his planet destroyed, he became a super sayian. Source: Bardock
My explanation isn't explained well enough, my bad guys.