Ultimate Super Saiyan is the highest form of Super Saiyan. Its power is so unbelievable even when restrained could tear apart a whole multiverse. Goku and gohan are the only ones to achieve this form.
This form's power is so high that nothing can beat it, possibly not even hurt it, which was proven wrong when he was easily beaten by Guardian, Vegeta as an Eternal Super Saiyan barely stood a chance.
This form has gray fur, increased body mass (similar to a buff form), and has silver hair that spikes up (similar to Super Saiyan 2).
Goku is the only one to ever obtain the form. The first time he ever used this was his battle against Eternal Super Saiyan Vegeta after he put on the Potara Earring. He fought only to be unharmed knowing Vegeta was not doing at his own free will. After about 5 minutes of fighting Guardian came to stop them. He easily dominated Vegeta and Goku using this form tried attacking only to last about a minute before also falling.
Battle against OverLord