He's gone through many transformations throughout the course of the Dragon Ball franchise, but undoubtedly, the most powerful form that Goku has reached thus far is Mastered Ultra Instinct and new fan art proves it.
Ultra Instinct itself is powerful enough as it allows the fighter's body to fight independently of any thoughts or emotions, just technique in its strongest state. Mastered Ultra Instinct, however, is Ultra Instinct in its most perfect, god-like state. The difference is that Mastered Ultra Instinct exceeds a power level higher than that of even Gods of Destruction, hardening the body to withstand insurmountable amounts of damage, with each strike of offense being the equivalent of unstoppable, divine, pure energy. Even compared to his previous Super Saiyan forms or even non-Saiyan-specific forms like the Kaio-ken, Perfected Ultra Instinct is by far the most powerful state that Goku has ever achieved. It is worth noting that the visual form of the Mastered Ultra Instinct is nearly as beautiful to look at as it is powerful. With his essence itself transformed into the personification of divinity, it is hard to argue that any of Goku's previous transformations surpass Mastered Ultra Instinct in terms of beauty and aesthetics.
That fact becomes especially true when looking at this dazzling piece of fan art by @spinekkop1 on Twitter. In highlighting the Z-Fighter's unexpected comeback performance during the Tournament of Power as a Universe 7 representative against Jiren, the Twitter user showcases Goku's Mastered Ultra Instinct in its most jaw-dropping and awe-inspiring presentation.
Perhaps what makes Goku's Mastered Ultra Instinct especially gorgeous is because it is a state in which readers have rarely seen him in. Not only is Goku depicted as the only mortal to achieve the form, as briefly as he achieves it, but he has only reached it a handful of times in the manga. While forms like Super Saiyan (and its many ascended forms) or Kaio-ken have been used several times over the course of the manga, the number of times Goku goes Ultra Instinct can still only be counted on one hand.
This makes Mastered Ultra Instinct feel more like a rare snapshot in time than another form of Goku's, which is why this fan art is so special. It takes the beauty of this snapshot in Dragon Ball history and intensifies it by highlighting the beauty of Goku's Mastered Ultra Instinct appearance.
Source: @spinekkop1