Throughout the course of his Dragon Ball career, Goku has only grown in strength and is currently wielding powers beyond that of a god. However, his first transformation is arguably his most powerful, not because it is the strongest, but because it literally broke the 4th wall–scaring even the creator of Dragon Ball himself.
Goku made his first appearance in Dragon Ball Chapter 1 by Akira Toriyama but his whole backstory wasn’t fleshed out until much later, revealing that Goku was actually a part of an alien race of warriors known as the Saiyans and he was sent to Earth as an infant just as his planet was on the verge of total annihilation. Once he reached adulthood, Goku pushed the limits of his power through rigorous training, with one of his first major achievements being Super Saiyan which was quickly followed by Super Saiyan 2 then Super Saiyan 3. During the events of the ongoing series Dragon Ball Super, Goku unlocked Super Saiyan God and even found a way to increase that divine power by reaching the Super Saiyan version of that, a transformation known as Super Saiyan Blue. Now, Goku has become successful in tapping into an angelic power known as Ultra Instinct, something Lord Beerus himself still hasn’t been able to master–but even all of those forms put together don’t hold a candle to his first transformation.
In Dragon Ball Chapter 21 by Akira Toriyama, Goku, Bulma, Yamcha, Oolong, and Puar have been captured by the villainous Emperor Pilaf and have been sentenced to death after foiling his plot for world domination. The cell the Z Fighters were kept in was completely impenetrable, including the glass ceiling that allowed them to look at the night sky as they awaited their impending doom. That night, there was a full moon and as Goku gazed upon its light, the Saiyan child suddenly transformed into a Great Ape–marking the first time readers ever saw this Saiyan transformation. On the final page, Akira Toriyama wrote a fearful note which read, “Gokuu, after looking at the full moon, has undergone a great transformation!! Even the author himself is surprised!! ‘It’s out of my hands!! It’s out of my hands!!’”
Goku’s transformation into a Great Ape was a shock, not just the characters in the manga but for Akira Toriyama himself! Toriyama was rocked by Goku’s monstrously powerful form. Goku’s later transformations, such as Super Saiyan and Ultra Instinct, are incredibly powerful and have granted him the strength he needs to conquer even the strongest foe, but none of them have been able to break the barrier between the real world and the fictional world in such an impactful way as Goku’s Great Ape form.
While Akira Toriyama clearly wrote the manga and assuredly wasn’t actually surprised by Goku’s transformation, the perspective the manga gives says otherwise. As far as the characters within the Dragon Ball universe are concerned, Goku’s Great Ape form literally broke the barrier between the fictional world of Dragon Ball and reality. Since the manga details the real-world reaction of Akira Toriyama within its fictional pages, Dragon Ball proves that Goku’s first Saiyan transformation is his most powerful since it was able to do what no other form has been able to: break the 4th wall.