The villains of Vegeta came to earth, he seemed like the biggest challenge the Z Fighters would ever face. Then came Frieza, Cell, Majin Buu, and many more, each one suring the other in strength.
However, none of them come close to Jiren's strength. The heroes did end up defeating him in the Tournament of Power, but that was only after wearing him down and teaming him up. There are many heroes Jiren could defeat alone, but some of them can defeat him.
Can Defeat: Master Roshi
In the Dragon Ball Super anime, Master Roshi and Jiren never crossed paths with one another. However, in the manga, the two actually fought each other. In chapter 39, when Goku is struggling against Jiren, Master Roshi steps in to let him rest.
Master Roshi manages to dodge Jiren's attacks, but only for a short time. Once he can't keep up much longer, Jiren knocks him out of the ring. In spite of being Goku and Krillin's teacher, he still wasn't powerful enough to defeat Jiren.
Can't Defeat: Whis
In Dragon Ball Super, Jiren is said to be a mortal who is stronger than a God of Destruction. Because of this, he is rumored to be able to beat his own God of Destruction, Belmod, and possibly Beerus. But that wouldn't make him strong enough to defeat Whis.
Whis is Beerus's attendant and teacher, which would make him stronger than the God of Destruction. Even if Jiren could defeat Beerus (which is questionable), he wouldn't be able to match Whis's speed and power, let alone outmatch it.
Can Defeat: Android 17
Android 17 was the ultimate winner of the Vegeta are down.
Unable to hold it off much longer, he decides to self-destruct to counter the blast. It took everything Android 17 had only to stop Jiren's blast. If just his blast could do that, Jiren would likely be able to do more in hand-to-hand combat.
Can't Defeat: Zeno
Zeno is the most powerful being in all of Dragon Ball; nobody comes close to his power. He's even helped Goku on multiple occasions, which does make him a hero of sorts. He defeated Zamasu, an immortal and powerful god, all without breaking a sweat.
He's even capable of destroying entire universes and its inhabitants in the blink of an eye. Given this, he'd surely be able to defeat Jiren faster than he could think about it.
Can Defeat: Gohan
Before the Tournament of Power, Piccolo.
This does allow him more access to his power, but not enough to defeat Jiren. Gohan's ultimate power is barely enough to overcome some of the opponents in the tournament, so it shouldn't be as challenging for Jiren.
Can't Defeat: Grand Minister
Being Zeno's attendant, the Grand Minister would have to be incredibly strong. The Grand Minister may have never been seen fighting in battle before, but Zeno likely wouldn't allow someone who would lose easily to watch so closely over him.
In addition to being Zeno's attendant, the Grand Minister is also the father of Whis and his several brothers and sisters. He likely taught Whis most of the things he knows about fighting, which would make him a pretty formidable opponent for Jiren.
Can Defeat: Vegeta
Because of his Super Saiyan Blue Evolution transformation, Vegeta was able to eliminate Toppo from the Tournament of Power. Toppo was training to become his universe's God of Destruction, so this wasn't a small feat. Even with this accomplishment, he wouldn't have what it takes to defeat Jiren.
In the episode "To the Noble, Proud End! Vegeta Falls!", Jiren knocks Vegeta out and eliminates him from the Tournament of Power. Even with his new form, Vegeta didn't have enough to take down Universe 11's proudest warrior.
Can't Defeat: Arale
Arale is from another manga by Akira Toriyama, called Dr. Slump. She has her own series, but she sometimes crosses over in the Dragon Ball world. She appears in Dragon Ball Super in the episode "Goku vs. Arale! An Off-the-Wall Battle Spells the End of the Earth?".
It's revealed that because she's a joke manga character, no attack can faze her. Goku and Vegeta try their hardest to slow her down, but nothing they do stops her. Eventually, she calms down and they leave on friendly , but that outcome probably won't be the same for Jiren.
Can Defeat: Goku
Goku has constantly broken through his limits, especially in the Tournament of Power. He even attained Ultra Instinct, something that's hard for gods to do. But without Ultra Instinct, Jiren still has the advantage over Goku.
In order to defeat Jiren, Goku had to team up with both Frieza and Android 17. On his own, the only hope Goku has of defeating Jiren is by fully mastering Ultra Instinct, and being able to activate it at will. Until that happens, Goku will never be the strongest mortal.
Can't Defeat: Gogeta
It's often debated who's stronger between Broly together. But when they fused into Gogeta, they made quick work of him.
In the Dragon Ball Super anime, Goku with Super Saiyan Blue/Kaioken teamed up with Super Saiyan Blue Evolution Vegeta against Jiren, and were able to hold their own against him. If they fused into Gogeta, which significantly magnifies their power, they'd stand a chance at taking down Jiren.
