Goku's Newest Dragon Ball Form May Be Stronger Than Any Super Saiyan
Through "Dragon Ball Z" and the start of "Dragon Ball Super," almost every meaningful power upgrade for Goku and his Saiyan allies is, in some manner, an iteration of the Super Saiyan form. Eventually, a few story arcs into "Dragon Ball Super," Goku does begin working on a new form called Ultra Instinct. His trainer, Whis, explains that with Ultra Instinct active, Goku will be able to react instinctually to his opponent's movements in battle. Attaining Ultra Instinct means raising his power level to a degree comparable to that of a god, and the road to perfecting the technique is appropriately difficult.
In the "Dragon Ball Super" anime, Goku achieves Perfected Ultra Instinct in his fight against Jiren at the conclusion of the Tournament of Power. But while his strength is considerable, the form rapidly drains his stamina, and he can't maintain it for long enough to win the fight decisively. Only in a storyline so far unique to the "Dragon Ball Super" manga, beyond just maintaining his Ultra Instinct form, Goku goes one step further and debuts an even stronger version that plays to his unique strengths.
As Goku explains in "Dragon Ball Super" Chapter 85 during his fight against Gas, Ultra Instinct normally requires a lack of strong emotion. Goku, however, figures out how to fuel Ultra Instinct with his emotions and becomes stronger as a result, greatly surpassing the strength of any Super Saiyan form in "Dragon Ball" history.
True Ultra Instinct is the strongest Saiyan transformation yet
Whereas most transformations throughout "Dragon Ball" are straightforward upgrades — Super Saiyan Blue, for example, is better in every regard than Super Saiyan God — Goku must first take a step backward to achieve his unique True Ultra Instinct form. His ability to freely indulge his natural emotions in this state is the result of activating an incomplete version of Ultra Instinct, called Ultra Instinct Sign, rather than the perfect version of the transformation. Because the imperfect form makes room for emotion, Goku can combine the benefits of Ultra Instinct with a Saiyan's inherent ability to grow more powerful in a heightened emotional state, becoming the strongest possible version of himself.
Perhaps the most blatant manner in which True Ultra Instinct differs from both Perfected Ultra Instinct and past Super Saiyan forms is its appearance. Goku's hair in this form is simply black like in his default state, rather than yellow, red, blue, or silver. Seemingly, then, True Ultra Instinct is the most natural expression of Goku's inherent abilities yet, and may well represent at the very least something close to the pinnacle of his fighting capabilities.