There are few characters in the Dragon Ball universe who take advantage of fusions like Goten and Trunks as they regularly become the Saiyan-amalgam fighter Gotenks while battling bad guys. However, while they arguably use fusions more than anyone, including Goku and Vegeta, Gotenks is far from the most powerful being created as a result of fusions–though he does have one attack that would have been perfect to use against the one character who actually is Dragon Ball’s most powerful fusion.
Gotenks made his debut in Dragon Ball Chapter 480 by Akira Toriyama. The form of Gotenks is achieved when Goten and Trunks successfully do the Fusion Dance which is a complicated technique that requires the two fighters to align their kis perfectly, then flawlessly pull off every intricate dance move to this mystical technique. As Gotenks, Trunks and Goten are able to simultaneously experience an incredible surge of power that neither could reach on their own. This includes their ability to pull off one powerful attack that has the strength to take on an entire army.
In Super Dragon Ball Heroes: Dark Demon Realm Mission! Chapter 11 by Yoshitaka Nagayama, alternate versions of the Z Fighters have joined an interdimensional police force, of sorts, known as the Time Patrol and they are on a mission to stop the Dark Empire before they rise up from their Dark Demon Realm and conquer the multiverse. In this chapter, the Time Patrol are going up against the entire Dark Empire Army and are greatly outnumbered. However, two members of the Time Patrol are Xeno Trunks and Xeno Goten, who are adults in this story, and they quickly fuse to become an all-new Gotenks. As Gotenks, the Saiyans use an attack called Super Ghost Kamikaze which effectively creates an army of ghost-like Gotenks that explode when they come in contact with an enemy.
In Dragon Ball Super’s "'Future' Trunks Saga" by Akira Toriyama and Toyotarou, Goku and Vegeta are going up against an immortal villain named Zamasu along with an alternate version of the villain who took over the body of Goku and became the nearly unbeatable fighter known as Goku Black. Zamasu is a god, and after making a wish upon the Super Dragon Balls, he became invincible. So, when he merged with his Saiyan-powered counterpart, the two separate villains became one and in doing so claimed the title as Dragon Ball’s strongest fusion. At the end of this saga, the Fusion Zamasu duplicated himself into hundreds of separate yet equally powerful entities in an effort to kill the Z Fighters once and for all and continue his dark quest of eliminating all mortal life in existence. While the Z Fighters defeat Zamasu with the help of Zeno, an all-powerful god, they would have easily died if the Omni King refused to assist them in their battle. However, if they had Gotenks with them during this battle, he could have used the Super Ghost Kamikaze attack to at least distract Zamasu long enough to fend off his attack as that one technique would have temporarily injured all of the Zamasus at once.
While Gotenks’ Super Ghost Kamikaze technique is not strong enough to kill Zamasu or any of his duplicates, his contribution to this battle would have given Goku and Vegeta the time they needed to formulate a plan of attack if Zeno refused to eliminate the threat for them–which would have made Gotenks responsible for the Z Fighters’ eventual victory. Because of his unique Super Ghost Kamikaze technique, Gotenks proves that he is the only hero (beyond an all-powerful god) who can conceivably beat Dragon Ball’s strongest fusion.