At this point in the franchise, Goku could easily defeat all of the villains from Dragon Ball Z without using any of his transformations.
At this point in the franchise, Goku could beat every villain from Dragon Ball Z without utilizing any of his transformations. And that’s because the events of Dragon Ball Super have taken Goku to new heights and have drastically increased his power level.
Four years after Dragon Ball Z ended, Goku’s life entered a new chapter with the arrival of Universe 7’s God of Destruction, Beerus. Goku’s battle with Beerus forced his fellow Saiyans to participate in a ritual that transformed him into the mythical Super Saiyan God. Though Goku ultimately couldn’t beat Beerus, he hasn’t given up on getting stronger. After training with Whis, Goku unlocked the Super Saiyan Blue form in the Golden Frieza Saga and discovered that he could amplify that power with his Kaio-ken technique in the Universe 6 Saga in the fight with Hit. During the Tournament of Power, Goku shattered his limits again by achieving a new form known as Ultra Instinct and overpowering Jiren, Universe 11’s strongest warrior.
Through Super Saiyan God, Super Saiyan Blue, and Ultra Instinct, Goku now has access to three massive power-ups that put him several levels above the adversaries he struggled against in Dragon Ball Z. However, Goku is so strong now that he wouldn’t need any of these transformations to defeat Dragon Ball Z’s villains. Frieza (at his peak in Dragon Ball Z), Perfect Cell, Dabura, and more would all fall to Dragon Ball Super’s base from Goku. Even Majin Buu, Dragon Ball Z’s most powerful villain, wouldn’t be able to hold his own against the Saiyan. This is true even when Buu was at his strongest after he absorbed Ultimate Gohan.
Goku demonstrated how effective he is in combat without transforming through several of his battles in Dragon Ball Super, one of which being his fight with the Vegeta clone. The clone struggled with Goku, but easily beat Super Saiyan 3 Gotenks (who is far above every Dragon Ball Z villain except for Super Buu). Also, Goku was able to spar with the good version of Buu, who is said to be more powerful than he was in the previous anime. In Dragon Ball Super’s Goku vs. Buu battle, Goku never needed to transform, even though he had to use his Super Saiyan 3 form to fight both Majin Buu and Kid Buu in Dragon Ball Z.
As for how this is possible, Whis explained in Dragon Ball Super that after becoming a Super Saiyan God, Goku can now use some of that divine energy without transforming. Another reason why he’s so powerful in this state is that Whis wouldn’t allow him to transform while training. This likely helped him to maximize his potential when in his base form. Looking at how formidable Goku can be without using any transformations raises questions about how much he could accomplish if he didn’t hold back. He already proved that he has surpassed Jiren, and by the end of Dragon Ball Super, it’s at least possible that he’s now capable of beating Beerus as well.