Despite knowing martial arts,
Starting off the list as Goku's weakest form is the original Dragon Ball transformation, the base Super Saiyan form. While still useful and powerful, even the mastered Super Saiyan form is the weakest of all the branch's transformations.
Krillin is often considered the weakest member of the Z-Fighters in Dragon Ball Z. However, he could easily defeat Perfect Cell and Frieza in a fight.
Arguably, Goku's greatest weakness is the fact that he never starts a fight going all out. He loves a good fight and has often held back just so whatever fight he's in can develop into a good one.
Emperor Pilaf is by far the weakest mainstream villain from the Dragon Ball franchise. However, Pilaf, Mai, and Shu were never meant to be "strong" in the conventional sense. Emperor Pilaf sought the Dragon Balls with plans of world domination.
Throughout the series, Vegeta's role changes from villain to later as one of the heroes, while remaining a key rival to Goku. Vegeta has been hailed as one of the most iconic characters not only in the Dragon Ball franchise, but also in manga and anime history as a whole.
1) Jiren. As seen in Dragon Ball Super's Tournament of Power arc, Goku was unable to defeat Jiren in a 1v1 even after achieving Mastered Ultra Instinct. While Goku has honed his Ultra Instinct since the Tournament of Power in the manga, Jiren and Goku have yet to rematch as Goku is now.
Vegeta is afraid of worms!
Beerus is an incredibly skilled and strong fighter as a God of Destruction. Even an Ultra Instinct Goku can't match his power, meaning a Super Saiyan God Goku has essentially no chance. The two had fought at the beginning of Dragon Ball Super, but the matchup was clearly in Beerus' favor.
It's no secret Dragon Ball's Yamcha is not a Saiyan, and neither is his shapeshifting accomplice Pu'ar. Yamcha is an Earthling, and was once a powerful and feared bandit who roamed the deserts.
1) Jiren. Jiren is undoubtedly one of the toughest opponents Goku has faced thus far in Dragon Ball. He forced the series' central protagonist into awakening his strongest form yet, one which allowed him to step into the realm of the Gods.
Super Saiyan 3 Goku at full power has a battle power of 24,000,000,000, 2 billion above Kid Buu's level.
Majin Buu and Lord Beerus are the Only Two Fighters Frieza Fears, but Maybe Not Anymore. Lord Beerus is a Destroyer God who has the power to eliminate planets without breaking a sweat and can even collapse the entire universe if he's fighting at full strength.
Due to his power and position, he is somewhat fearless, as there are very few who can match his power, let alone capable of killing him in combat. However, Beerus is extremely terrified of Zeno, in a manner very similar to how the Kais and other gods are scared of Beerus himself.
I hear pretty commonly from people that Gohan's characterization in the Cell Games came from a fear of unleashing his potential. He was so afraid of unleashing it and possibly hurting his friends that he didn't want to take action, or so the theory goes.
Put simply, Goku can't be killed with the Death Note, as its power, and the power of the Shinigami over the realm/state/process of death by extension, are only effective over human beings.
As the main character, Goku eventually surpassing Beerus is all but guaranteed. After all, Goku's journey is one of encountering and overcoming obstacles. It was demonstrated with Demon King Piccolo, then Frieza, then Kid Buu. Beerus is yet another obstacle for Goku to overcome.
Goku has a ton of potential to surpass several gods. With Ultra Instinct Sign, and who knows what other transformations, it is clear he will continue to get stronger until he breaks through yet another limit!
Indeed, Goku has been beaten countless times, and it's usually actually a tradition to have him lose against an arc villain before training and getting powerful enough to defeat our villain once and for all.
Gohan always had the most potential out of all the Z-Fighters, but his lack of training and commitment always hindered his growth, while Goku instead keeps getting better. According to Piccolo, however, Gohan can still surpass his father.