Although one can debate anything, there’s very clearly a right answer as to Goku’s strongest form as of 2021. Here's a look at the evidence and the arguments throughout Dragon Ball that can be made to crown a clear winner.
What is Goku's strongest form as of 2021?
Super Saiyan God and Super Saiyan Blue
Super Saiyan God and Super Saiyan Blue are both techniques that focus on Saiyans achieving power similar to gods. This is done by channeling God Ki instead of regular Ki, which is then used to transform the body in a way similar to Super Saiyan. Despite laying claim since debut, neither is what Goku's strongest form is as of 2021.
While Super Saiyan God requires a bit more explanation, the above summary suffices for this argument. Clearly, Super Saiyan God and Super Saiyan Blue were great steps in Goku and Vegeta’s journeys of strength. This is the first step our protagonists take into the Realm of the Gods. Although strongest at the time, these, unfortunately, aren't what is Goku's most potent form as of 2021.
While an excellent base for their God-like strength, foundations are meant to be built upon for a grander final product. Goku does so in the Universe 6 saga, where he debuts a new form by combining it with a throwback. Unfortunately for Super Saiyan God forms, Goku's new technique based on them, shows they're not Goku's strongest form as of 2021.
Super Saiyan Blue Kaioken
The Super Saiyan Blue Kaioken made its debut when Goku faced Hit in the Universe 6 vs Universe 7 tournament. Able to combine the two forms because of the God Ki in his body, Goku finally is able to do something he himself said was never possible before. With the power it boasts, it must surely be a contender for Goku's strongest form as of 2021 for many fans.
It’s great to see Goku foundationally building on Super Saiyan Blue, finding new ways to improve its strength and power output. Although there are still severe physical consequences for combining the techniques, Super Saiyan Blue Kaioken is a fantastic form that both makes sense and hypes the fans up. However, is this what is Goku’s strongest form as of 2021?
Unfortunately, this is not what is Goku's strongest form as of 2021. Our hero still has greater heights to climb, which Goku does begin to ascend during the Universe Survival saga. From here, fans will start to see what is Goku's strongest form as of 2021.
Imperfect Ultra Instinct
This form begins scratching the surface of what Goku’s strongest form is as of 2021. Imperfect Ultra Instinct debuted during a Ki exchange between Goku and Jiren, when Jiren sends a Spirit Bomb back at Goku. Goku attempts to absorb the Spirit Bomb, which he does, albeit while taking damage.
Goku springs back to life, successfully absorbing the Spirit Bomb and using the massive influx of energy to activate Imperfect Ultra Instinct. MUI Goku fights on relatively even ground with Jiren, with neither getting significant offense in. This begins scratching the surface of what is Goku's strongest form as of 2021.
We then see Goku show off Imperfect Ultra Instinct by eliminating Kefla, via one of the greatest Kamehameha’s Dragon Ball fans have ever seen. Imperfect Ultra Instinct sadly goes away after this, but fans can clearly see Goku now has something to build towards. This ascension is what is Goku's strongest form as of 2021.
Mastered Ultra Instinct
Mastered Ultra Instinct, the perfected form of the above transformation, is easily what is Goku’s strongest form as of 2021. Activating it in the final moments of the Tournament of Power, Goku gives Jiren a run for his money with this form. We see Goku effortlessly deflect Jiren’s attacks while simultaneously landing his own.
The final fight between MUI Goku and Jiren is fantastic, and fans can see it as Goku's strongest form as of 2021. Even an enhanced version of Super Saiyan Blue couldn’t stand up to Jiren, as showcased by Vegeta in episodes prior. Considering Super Saiyan Blue is the only other form an argument can be made for, MUI is clearly Goku's strongest form as of 2021.
Unfortunately for fans (but fortunately for the storyline), Goku is still unable to activate MUI on command. Goku needs to train with Whis more until he can use it inherently without even activating the form. Once achieved, MUI will inarguably be Goku's strongest form as of 2021.
Although fans are asking what is Goku’s strongest form as of 2021, the answer is very clear and obvious as Mastered Ultra Instinct.
There’s no debate that the Mastered Ultra Instinct is Goku’s strongest form as of 2021. It’s established as more powerful than Super Saiyan Blue, Super Saiyan Blue Kaioken, and Super Saiyan Blue Evolved. While some of these are established via the transitive property, they can still be used to deduce Goku's strongest form as of 2021.
There just currently isn’t a form yet that can truly answer the question of what Goku’s strongest form is as of 2021. Mastered Ultra Instinct is just too clearly above everything else in Dragon Ball right now, and it is easily what is Goku’s strongest form as of 2021 and beyond.