Among the most powerful fighters in the entire Dragon Ball franchise are the formidable twins Androids 17 and 18. Despite being referred to as "Androids" -- with the original Japanese manga and anime series referring to them as "artificial humans" -- the two warriors are actually humans who were cybernetically enhanced by the fiendish Doctor Gero as part of his obsessive vendetta against Goku. Androids 17 and 18 continue to play a major role in the Dragon Ball franchise, with 17 playing an especially prominent role in Dragon Ball Super. Here's why Gero selected the siblings to become his most lethal creations in his plot to destroy Goku.
The Daizenshu, the officially licensed encyclopedia for the Dragon Ball franchise, along with ancillary material in the colorized edition of the Dragon Ball Z manga, reveals that Androids 17 and 18 were twin siblings Lapis and Lazuli respectively. In keeping with Akira Toriyama's tradition of using puns to name his characters, Lapis Lazuli refers to the precious gemstone of the same name as a nod to the twins' striking good looks compared to other Dragon Ball characters. Lapis and Lazuli's family background is unknown, although the twins had gained a notorious reputation for being juvenile delinquents before they even met Gero.
The Daizenshu reveals that the twins encountered Gero purely by chance, with the mad scientist deciding on the spot that the two would be perfect subjects for the cybernetics experiments he was carrying out in secret to develop the perfect warrior. Gero intended for his creations to kill Goku in retaliation for the young Saiyan dismantling the Red Ribbon Army -- a criminal organization of which Gero was a committed member in the original Dragon Ball series. Kidnapping the twins, Gero reasoned that he could modify his enhancements for the second sibling based on their reaction to the first twin he experimented on, as their genetic makeup was so similar, effectively getting two chances at the same experiment.
Gero's decision to obtain twins to see how they reacted to his cybernetic enhancements led to subtle differences between 17 and 18 upon the completion of his experimentation. The Dragon Ball Super manga series reveals that Lapis was experimented on first by Gero and deemed to be too insubordinate to Gero's commands. Taking this data into account, Gero decided to make Lazuli's cybernetic enhancements slightly weaker, theoretically making her easier to control than the brother at the expense of raw power. The power difference between Androids 17 and 18 was evident during Dragon Ball Super's climactic Tournament of Power, with 18 eliminated from the martial arts competition considerably faster than her brother, who ended up emerging triumphant while revealing himself to be roughly as powerful as a Super Saiyan Blue.
Ultimately, Gero discovered that both Android 17 and 18 were too difficult for him to completely control, and he put both siblings into stasis while he continued his robotics experiments. Gero's perceived failures with transforming Lapis and Lazuli led to him making Android 19 a completely synthetic, albeit noticeably weaker warrior, while Gero himself transferred his own brain into a synthetic body to become Android 20.
Gero's concerns about controlling 17 and 18 would be justified, with the twins killing him immediately upon their reactivation. However, given the fact that the evil scientist kidnapped and experimented on children all over a petty vendetta, Gero certainly had cybernetic justice coming his way for what he did to Lapis and Lazuli.