Ultra Instinct is not the highest form of Super Saiyan, because it's not a Super Saiyan form at all. It's a technique, a Zen state that allows for movement without conscious thought.
According to an interview, Akira Toriyama designed Ultra Instinct — including its name — as a power-up for Son Gokū that would be distinct from the Super Saiyan form. It was only decided after the flow of the Universe Survival Arc's story had been decided on.
Through training, Goku developed the ability to utilize the Ultra Instinct Sign at will. Merus points out that at that point Goku was just one step away from achieving the full form and when he does he will be able to access it whenever he wants, and thanks to having become stronger, he will be able to keep it stable.
Whis explicitly states that while Goku is using it as a transformation, Ultra Instinct is in fact not a transformation, but a high level divine technique. Anyone can achieve it, but it's really hard to even learn the basics.
Super Saiyan 4 eliminates this by giving Goku access to all that power with control. The Ultra Instinct form, which is the most recent transformation Goku has experienced, is less of a physical transformation, and more of a mental state of mind.
Unlike Ultra Instinct, Ultra Ego can be tapped into easily by the user once they awaken it completely. As seen during the Tournament of Power, Goku struggled to do that with his ability, and it wasn't until much later in the manga that he finally mastered it.
Prior to the Galactic Patrol Prisoner Saga, Goku could not access Ultra Instinct on his own and it only activates when he is in the direst of situations. Goku achieves the ability to use Ultra Instinct Sign at will during his training with Merus, a trainee Angel who also possesses the Ultra Instinct ability.
A final and most likely scenario is that there's simply nothing after Ultra Instinct. Goku will eventually learn Autonomous Ultra Instinct, and this will be his most powerful form through the rest of Dragon Ball.
1) Ultra Instinct Vegeta
He even seemed to tap into it for a brief moment, though in hindsight, this seemed to be luck or basic battle instinct. With the introduction of Ultra Ego, and confirmation from both Vegeta and Beerus that Ultra Instinct wasn't his style, he'll likely never achieve the form.
Goku's final form is his True Ultra-Instinct form which was initially introduced in Chapter 85 of Dragon Ball Super Manga ''To Each His Own Answer''.
They call this form "真の身勝手の極意/True Ultra Instinct". (Source: dragon-ball-official.com/news/01_1272.h…) A few hours ago, Dragon Ball's official site published a summary of the Granolah Survivor arc up until Chapter 85. It was in this chapter that Goku was able to access this new form.
With 21 canon variations of Super Saiyan now existing, here is a breakdown of all Super Saiyan forms in Dragon Ball.
Beerus has access to both Ultra Instinct and Ultra Ego and to make him even more overpowered, he has the power of Hakai.
Though Vegeta has trained with Whis, he told Jiren that he has “no master” and that his power was forged by “the fires of solo training”. He explained that he doesn't like being taught anything, which means that he wouldn't want to learn Ultra Instinct from Goku or Whis.
Ultra Instinct is based of Mushin which means “no mind”. Goku's mind was exceptionally calm while he was using Ultra Instinct, and was barely thinking, hence the silence.
Thus, it is safe to say that Limit-breaker Saitama cannot beat Goku in his Ultra Instinct form. It is far too powerful a transformation to contend with. Even if Bald Cape lands his Zero Punch, Goku has been seen multiple times being more than capable of taking a hit.
Super Saiyan 3 Goku at full power has a battle power of 24,000,000,000, 2 billion above Kid Buu's level.
Perfected Ultra Instinct is known by other names such as Mastered and Completed Ultra Instinct, and is still the apex of Goku's power. Not only did Goku beat Jiren in this form, but he also defeated Moro as well.
While Dragon Ball's Goku has unlocked god-like powers of his own, he still doesn't stand a chance against Lord Beerus, the God of Destruction if the two were to ever fight again. There is, however, one classic Dragon Ball hero who actually could fend off Beerus with one ingenious attack.
Vegeta first attains Ultra Ego through the Ultra Ego transformation against Granolah after his Super Saiyan God SS Evolved is overwhelmed by the Cerealian. The transformation was powerful enough to cause Granolah to stumble back in noticeable fright.
Hercules is the Olympian God of Strength, and he might be one of the unique ones on this list capable enough of matching and overpowering Ultra Instinct Goku in terms of pure strength. Being a Half-God, Hercules is nigh-omnipotent and invulnerable to almost anything.
Gohan Beast is Gohan's most powerful new form. Unlike the more straightforward Super Saiyan God forms, this one is based on Gohan's more animalistic transformations that he had as a child.