Goku is the most famous "Dragon Ball Super" character right now, well in almost all Dragon Ball franchise actually. As the main hero, he gets a lot of transformation, and most of the time, he discovers them. With his forms growing in number in the current DB anime, fans are debating what form is his strongest form so far.
One of the most badass (and most unusual) transformation Goku has yet in "Dragon Ball Super" is the Super Saiyan God. It is the form that he got by a righteous ritual of six Saiyans. Den of Geek elaborated that the ritual is power consuming that the participants except Goku are not able to even fly after. In this transformation, Goku is simply a God.
Goku stood a chance versus Beerus for a while until the God of Destruction upped his attacks. It seems that this form is Goku's go-to when he get a lot of time to prepare for a battle, like for example, the Cell Games. Had he mastered this form before fighting Beerus in "Dragon Ball Super", things could be more interesting.
Goku's other form is the Super Saiyan Blue Kaioken which first appeared in his battle with Hit. According to Attack Of The Fanboy, this technique makes Goku tired which reduces his combat time. This transformation simply put is a Kaioken topped on Super Saiyan Blue. As it is a "burst" kind of power-up, it consumes a lot of energy and will damage Goku's body in the long run. This form might be seen only in short intense battles in "Dragon Ball Super".
Against Hit, this makes it effective for Goku to counter time manipulations. This form will likely not appear in a Toppo VS Goku fight as Toppo is not only strong, but he is also durable. Just like the Great Priest said when he's stopping the fight, one might end up dead, and that one is likely Goku because of the strain of SSB Kaioken. It is seen in "Dragon Ball Super" episode 82 that Goku is equally strong as Toppo. To beat Toppo, Goku might develop another form. It might be an upgrade to Super Saiyan Blue, like a Super Saiyan Blue 2, or Super Saiyan Blue Second Grade.