One of the most deceptively powerful characters in all of Dragon Ball isn't among the usual choices like series leads Goku and Vegeta. It isn't Gohan, with his extraordinary amount of untapped potential or Piccolo with his esoteric Namekian abilities. Instead, it's Android 17, with untold levels of power and capabilities that make him one of the most formidable warriors in the entire Dragon Ball Multiverse as confirmed by the end of Dragon Ball Super and hinted at, with far more sinister implications, in the non-canonical series Dragon Ball GT.
Android 17, along with his twin sister Android 18, was created by Doctor Geroas as the ultimate killing machine to eliminate Goku and avenge the Red Ribbon Army's Dragon Ball-era destruction. 17 proved himself superior to the Z Fighters right from his debut in Dragon Ball Z. His cyborg body meant 17 never tired, giving him a visible edge over Piccolo, even after the Namekian champion merged with Kami to become the strongest Namekian in Universe 7. And while 17 was overpowered by Cell during their first meeting, he would resurface stronger than ever in the middle of Super, becoming Universe 7's unlikely savior during the Tournament of Power.
As Goku searched for recruits to represent Universe 7 in the tournament to decide the fate of the multiverse, he approached 17 after not having seen the android since the Cell Games years prior. After a friendly sparring match to see if 17 should have a spot on the team, Goku was surprised that 17 could more than keep up with him after all this time, even after Goku went Super Saiyan Blue. 17 mentioned that, while working at a remote wildlife preserve, he had quietly kept training ever since his resurrection via the Dragon Balls after the Cell Games. 17's significant increase in combat strength proves vital to Universe 7's victory in the Tournament of Power, with 17 shocking everyone as the last fighter standing after a prolonged battle against Universe 11's champion, Jiren.
Dragon Ball GT provides a considerably darker character arc for 17, with the android resurfacing a year after Baby's defeat. While in Hell, Gero and Doctor Myuu team up to construct a doppelgänger of 17, aptly named Hell Fighter 17, to contact his counterpart on Earth and open a portal allowing those in Hell to escape. Hell Fighter 17's presence corrupts the original android, causing him to attack the Z Fighters, before merging to form the powerful Super Android 17. Capable of withstanding Super Saiyan 4 Goku's mightiest blows, it took the original 17 taking control of his own merged body long enough to make himself vulnerable for Goku to strike and ultimately save the day.
Incapable of fatigue and possessing unlimited reservoirs of energy, Android 17 has risen through the ranks by the end of Dragon Ball Super to become one of the strongest fighters of all. Not even Goku can maintain the Ultra Instinct transformation for a prolonged period of time and, while stronger than 17 in this form, 17 could theoretically wait him out by evading the Saiyan fighter. And with 17 exponentially stronger from training for a decade, there's no telling how much more powerful the android could become if he spent that same time training in the Hyperbolic Time Chamber -- he could emerge the strongest Z Fighter in the entire multiverse.