Most of the villains in Dragon Ball follow similar patterns. While the best among them have enough personality and style to stand out, they all tend to be heartless killers who want to destroy anything that stands in their path. That's what makes the few exceptions stand out so much, and no character embodied them more than Android 16. Designed to kill Goku, the character has a minor subplot throughout the "Android" and "Cell" Sagas that elevates him above almost any other antagonist in the franchise.
Android 16 quietly brings a sense of somber morality to the largely fight-first Dragon Ball Z and is a perfect tragic figure for this world.
WHO IS ANDROID 16?
The Red Ribbon Army was among the many enemies who confronted a young Goku during the events of the initial Dragon Ball series. And like the rest, they came up short. Doctor Gero helped devise the weapons and androids that the Red Ribbon Army used in their conflicts. While a majority of the army was wiped out by the young hero and his allies, Gero survived and went into hiding. Gero resumed his work on creating the ultimate fighting machine. Using a device, he was able to study the fighting techniques of Goku and his various opponents over the years -- eventually even studying the DNA of the universe's greatest warriors to make his greatest creation, Cell. Along the way, he constructed various other androids to serve as his new frontline of soldiers.
Among them were Android 16, modeled after Gero's son (who'd died in an undisclosed battle as a soldier in the Red Ribbon Army). He was an extremely powerful being, but incomplete at the time of Gero's attack on Goku and the others. Even when Gero was desperate enough to activate the rebellious Android 17 and Android 18 he hesitated to activate 16. The pair activated him after killing Gero and were surprised to find him to be a surprisingly soft-hearted. His only violent desire was to follow Gero's programming and kill Goku. Besides that, he didn't have any violent impulses.
He forged a friendship with the other two Androids, and even eventually joined in the battle against Cell. However, he was quickly outclassed by the powerful organism. After being repaired by Bulma, he attempted to destroy Cell with a bomb built into his chest -- unaware that Bulma had removed it out of fear of 16 using it to try and kill Goku. Cell was able to use this opening to instead destroy 16's body and the android spent the final moments of its life talking to Gohan, motivating him to fight the villain and save the world.
SOUL IN THE MACHINE
Android 16 considers the continuation of life on Earth to be one of his prime missions. Despite his robotic design, Android 16 proves to be calm and friendly by his very nature. He's a subversion not just for the other chaotic and destructive Androids but most of the villains in the entire franchise. He even positions himself as the direct opposite against Cell, who is designed to wipe out all other forms of life and absorb it into himself. As opposed to other villains in the franchise, Android 16 is driven more by the same protective desires that fuel most of the Z-Warriors and their drive to fight for Earth. The truth is, he should have been a hero all along.
But he's still nominally a villain. His specific designs were to try and kill Goku, no matter what. Even when he is somewhat redeemed by his mission to protect the natural life that he cares so much about, that intention to bring down Goku once and for all is at his core. In some videogames like Dragon Ball: Xenoverse 2, it's shown that Android 16 even recognizes that Goku is a good and noble man, and that he would prefer if his drive wasn't to murder him. It separates Android 16 from the other villains in the franchise, giving him a surprising amount of regret and humanity despite his robotic design. He's just genuinely kind, which doesn't much exist within the battle heavy Dragon Ball universe.
AN ENDING
What really separates Android 16 from the rest of the characters within the Dragon Ball franchise, though, is the sense of finality that comes with him. The presence of multiple Dragon Balls across the universe means there are plenty of opportunities to resurrect the core characters after they've died. In fact, by the events of Dragon Ball Super pretty much every single character has been killed and restored by magic at least once. It means that the tension of battle is undercut throughout the show. While there are workarounds to make death a threatening concept again (such as limits on how many times someone can be restored, a rule that becomes weaker as the story progresses), no one confronts it so head on as Android 16.
Since he was fully constructed instead of being a human/machine hybrid like Android 17 and 18, it's impossible for the dragon to bring him back to life after he's killed by Cell. He was never technically alive, after all. Even when the character appears again in videogames like Dragon Ball FighterZ, it's made clear this isn't the same Android 16 but merely a new model that considers himself a "copy" of the original. The original Android 16 is among the only characters in the entire franchise to die and then actually stay dead, making his poignant final moments with Gohan all the more powerful in retrospect. He's not only a subversion of many elements in Dragon Ball but the perfect somber and conflicted bad guy for a series that usually makes villains outlandishly evil.