Goku and the Z Fighters have faced off against increasingly more powerful opponents, and their biggest antagonists yet were introduced in the most recent anime series, Dragon Ball Super. The final challenger to take on Goku and the rest of the team representing Universe 7 in Super's climactic Tournament of Power was Jiren. A Pride Trooper from Universe 11, Jiren possessed combat power comparable to the gods themselves as the strongest single fighter in the Dragon Ball Multiverse appearing at the tournament.
Set after the events of the anime series, the franchise had released the anime feature film Dragon Ball Super: Broly, providing the canonical introduction of fan-favorite villain Broly. Broly previously debuted in a trilogy of non-canonical anime films and video game adaptations set during the Dragon Ball Z era. The incarnation of the Legendary Super Saiyan in the Super film was even stronger than he had been before, now capable of easily overpowering Goku and Vegeta even after both Saiyans had transformed into Super Saiyan Blue.
But which of the two hulking antagonists would emerge victorious should they never confront one another? After all, both Jiren and Broly survived their respective battles against Goku, with both vowing to grow even stronger for an inevitable rematch, so it's entirely likely that a showdown between the two could actually occur in a future adventure. And with Broly able to stand toe-to-toe against Gogeta even with the composite warrior at Super Saiyan Blue, he could likely be Jiren's toughest opponent yet.
One thing that's important to note is that fighting against Jiren pushed Goku far enough past his normal physical limits that it triggered the longtime Dragon Ball protagonist to transform into his Ultra Instinct state. While Goku was unable to defeat Broly alone, he was never under enough sustained physical duress for the Ultra Instinct transformation to trigger—this suggests that Jiren may be the more challenging opponent for Goku. The other side of that is perhaps Broly's Legendary Super Saiyan state didn't give Goku the opportunity for Goku to transform, whereas Jiren had intentionally held back for much of the tournament.
Broly steadily grows more powerful as he fights, so Jiren would have to take a potential match-up very seriously from the start to prevent Broly from rising to his level and possibly overpowering him. Possessing strength equal to a God of Destruction certainly gives Jiren an edge in this regard—in the Super film, Broly was seen unable to land a single blow on Whis who had naturally achieved defensive capabilities similar to Ultra Instinct. Broly, like all Saiyans, is powered by sheer rage, and should he go into a fight angry, he may catch Jiren off-guard. While engaging in a normal bout, however, Jiren has a definitive edge.
Broly and Jiren are the two strongest opponents Goku has ever fought, likely stronger than Beerus himself judging by the God of Destruction's obvious discomfort watching Jiren fight. However, with a lifetime of rigorous training and nigh unflappable combat discipline, Jiren has a definitive edge against the more raw, uncontrollable Broly. Jiren would have to put down the Legendary Super Saiyan quickly if he wanted to walk away from the truly epic battle, and he could never take his opponent for granted—which, admittedly, was a major flaw of his during the Tournament of Power. Against such a savage warrior as Broly, to falter even for an instant, even at Jiren's level, could mean a crushing defeat.