Android 17 appears in three series with three wildly different roles: in Dragon Ball Z he is the rebellious android who treats hunting Goku as a game, in GT he becomes his creators' weapon, and in Super he is a park ranger who decides the Tournament of Power. His full story is in Android 17's profile; here we retrace it arc by arc.
Dragon Ball Z
Androids Saga (episodes 133-139)
In the Androids Saga a cornered Dr. Gero wakes Android 17 and his sister Android 18 as a last resort. In episode 133 17 is defiant from the start; soon after, he crushes the remote Gero could use to shut him down and kills his creator. Together with 18 he also activates Android 16, and the three set off to find Goku. When Vegeta takes on 18, 17 stops the others from interfering, and in episode 136 he flattens Future Trunks, Piccolo and Tien. To Krillin, the only one left unhurt, he explains that killing Goku is just a game to them; then the three drive off, for the fun of it, in a stolen van.
Cell Saga (episodes 146-194)
They find Goku's house empty, so the androids move on to Kame House. In the Cell Saga Piccolo, freshly merged with Kami, takes 17 to a deserted island, and in episode 148 the two turn out to be evenly matched: 17 has infinite energy, Piccolo has regeneration. The fight, however, draws Cell in. 17 refuses to be absorbed and Android 16 tries to defend him, but in episode 152 Cell catches him from behind and swallows him with his tail, becoming Semi-Perfect. 17 dies when Cell blows himself up on King Kai's planet, and comes back thanks to the wish with which Shenron revives Cell's victims; a second wish from Krillin removes the bomb hidden in his body and his sister's. In Trunks' future, by contrast, 17 and 18 have ravaged the Earth for years: in episode 194 Trunks returns home far stronger and destroys them.
Majin Buu Saga (episode 285)
17 also dies in the attack Super Buu uses to wipe out humanity, and comes back with the wish made to Porunga for the Earth. In the Majin Buu Saga he only gets a cameo, but a meaningful one: in episode 285, recognising Goku's voice, he is among the few who raise their hands straight away to give energy to the Spirit Bomb.
Dragon Ball GT
Super 17 Saga (episodes 42-47)
Down in Hell, Dr. Gero and Dr. Myuu have built a copy of 17 and are controlling the original from afar. In the Super 17 Saga 17 attacks Trunks, hypnotises 18 and, when Krillin brings her back to her senses, kills him. In episode 44 the two 17s merge into Super 17, who knocks out Vegeta, Gohan, Goten, Trunks and Majuub and, on Myuu's orders, kills Gero once more. Against Super Saiyan 4 Goku he absorbs every energy blast, until 18 shows up: Super 17 destroys Myuu, and Goku realises he can't move while absorbing energy. In episode 47 Goku punches through him with the Dragon Fist and finishes him with a Kamehameha. 17 is revived by Shenron's last wish at the end of the series.
Dragon Ball Super
Universe Survival Saga (episodes 83-131)
Goku and Gohan put him on the list of fighters for the Tournament of Power, and Dende helps Goku find him. In episode 86 Goku tracks him down on an island where he works as a ranger, protecting the animals from poachers; he has a wife and three children, two of them adopted. The two have a friendly sparring match, but 17 turns the offer down. He only changes his mind after he and Goku repel a ship of space poachers, on condition that Goten and Trunks watch over the island while he is away.
In the Universe Survival Saga he fights alongside his sister against Universe 2; 18 sacrifices herself to keep him in the ring, and he strikes the reactor of the giant Agnilasa so the team can knock it out. With Frieza he takes on Toppo, then in episode 127 he shields Goku and Vegeta with his barriers and seems to blow himself up to stop Jiren's attack. In fact he survived: he reappears in the finale, covers Goku and Frieza as they drag Jiren off the stage, and in episode 131 he is the last fighter standing. With the Super Dragon Balls he wishes for every erased universe to be restored.
Did you know? In Toriyama's rough draft of the manga, the character raising his hand in Android 17's place during the Spirit Bomb scene was Launch, as the author revealed in his notes for the Full Color edition.
Our take
Followed arc by arc, Android 17 is the Dragon Ball character who has gained the most from time. In the Cell Saga he is fascinating but used up quickly: he kills his creator, fights Piccolo to a draw and gets absorbed without ever crossing paths with Goku. GT turns him into someone else's tool, without giving him a real voice.
Super gives it all back: a life chosen far away from fighting, a family, and victory in the most important tournament of the series, earned through stamina and cool-headedness rather than a transformation. His final wish, generous and unhesitating, closes the arc in the way that best fits the man he has become.
For the monster who absorbed him see Cell saga by saga, for his most unpredictable teammate Frieza saga by saga, and for his twin sister Android 18's profile.