Some people say Jiren is stronger than Broly, while some people say Broly is stronger than Jiren. Usually, in Dragon Ball, the next enemy is stronger. However, this pattern was changed in Dragon Ball Super as Beerus is stronger than most of the antagonists that came after. So who is stronger of the two?
Dragon ball generally follows a pattern where the antagonist tends to be > than or around the level of the protagonist. If you go back to Dragon Ball Z, Super Buu is stronger than Kid Buu and Buuhan and Butenks are way stronger than Super Buu. However, Kid Buu was the final antagonist.
The antagonist's power is generally adjusted to be comparable to the protagonist they are fighting. Since Ultimate Gohan is stronger than Super Saiyan 3 Goku, they had him fight Super Buu. The same goes for Vegito, who fought the strongest iteration of Buu. Just like you said, despite Beerus being stronger than the rest of the antagonists after, they did use a greater amount of power against Goku and co.
However, after watching the movie, I personally believe there is sufficient reason to believe that Jiren might be stronger( At least the version of him where he releases his dormant power). There are several reasons why I believe this to be true:
- The Tournament of Power emphasizes the fact that Goku surpasses the Gods and manages to overcome Jiren. This was constantly emphasized during the course of the tournament while he was Master UI. Jiren was able to go toe to toe with this iteration of Goku and also come out on top in terms of raw power. When the trailers and initial summary of the movie first came out, it was stated that Goku was almost reaching the level of a God and Vegeta was catching up(This makes sense because it was established at the end of DBS that Goku does not have access to Ultra Instinct anymore).
- Broly as a Super Saiyan was no doubt stronger than Super Saiyan Blue Goku and Vegeta. However, there isn't a long enough fight to determine the exact difference. Goku and Vegeta would have definitely grown stronger compared to the T.O.P. However, they were using forms 20 times stronger against Jiren who was nowhere near full power while fighting these two. Also, Golden Frieza managed to last against an agitated Broly who was looking to kill. He barely lasted for a second against Jiren who took him out with a glare and a single punch
- In the tournament of power, it was more or less implied that the only way to defeat Jiren was for Goku to master Ultra Instinct. If Goku and Vegeta could've defeated Jiren with fusion, it would break the entire plot and the entire struggle against Jiren wouldn't make sense. This is because Vegeta had absolutely no problem working with Goku and doing everything to win. It was stated that Fusion was allowed and they both saw a potara fused a character a while back. Not even Whis or anyone stated Goku and Vegeta would be able to win this fight fused. So I do believe Jiren at the time was stronger than Vegito, hence Gogeta(Definitely when he releases his dormant power). SSJ Broly had a disadvantage against SSJ Gogeta. LSSJ Broly could barely hold his own against SSJB Gogeta.
- Finally, at the end of the movie, Goku compares Broly's power to that of Beerus. However, Goku and Vegeta mentioned that Jiren was the strongest they ever faced when they fought him, implying Jiren was > Beerus.(Goku says a suppressed Jiren's punch was the strongest he's ever faced, Vegeta says he hasn't felt a level of energy like this before). The V-Jump magazine had an article where Beerus comments that Goku has surpassed him after achieving UI, which you can see here.
- In the Battle of Gods arc, SSJG Goku was said to be stronger than Vegito. If you compare the difference between SSJ3 and SSJG, the power is significant indeed. However, the difference in power is a lot less if you were to compare SSJB + Kaioken * 20 To UI Omen, the difference in power is a lot more significant, and Mastered Ultra Instinct is substantially stronger than the two. Since the potara multiplier is fixed, I believe MUI Goku would still be stronger
In conclusion, the next arc of the manga states that Goku was still attempting to master UI as he tells Vegeta that he hasn't been able to tap into the transformation even once after the tournament of power. So if the series were to resume, Goku might most likely tap into this form against a stronger Antagonist than even Jiren and Broly. There is promotional material about Broly's strength claiming how he is one of the strongest and they say his level of destruction might be greater than that of a God of Destruction. The source for this can be seen here. However, there is no legitimate valid source indicating Broly to be stronger than Jiren. DBS did something new in the series by having Goku achieve two transformations in 1 arc and at the end of the arc, not be able to tap into it anymore. I think this on its own is sufficient evidence as to how Over Powered this iteration of Goku is and why the plot decided to take this transformation away before bringing characters even stronger. Plus, Whis's training is mainly focused on making them achieve the form. If Goku did achieve it, he would technically have completed Whis's training which is necessarily not the case based on the beginning of the movie.
There is no conclusive evidence in my opinion but, I think it's Jiren because of this mainly, the Broly movie apparently takes place shortly after the tournament of power, and Freezer was able to resist being beaten by Broly for 1 hour when in the tournament of power Freezer was seriously damaged by Toppo, who is at least 2 levels behind Jiren full power (Toppo is weaker than Jiren not using full power).
Aside from that, Goku thinks Broly "might be stronger than Beerus" which in my opinion means he's close to Beerus's power (since Goku isn't sure) when Jiren is said to be stronger than gods of destructions when Jiren was almost a match for Goku mastered ultra instinct (a state to which all the gods of destructions almost vowed to, they stand up when Goku transformed as a sign of respect or admiration, like admitting it was a state superior to them, Beerus had to ask Wiss about Goku's power because it was beyond his comprehension, Beerus sweated to Goku Ultra Instinct Omen which was almost a match for Jiren not in full power and the other gods of destructions were surprised, etc.)
Eventually, we could get an answer in some of the Dragon Ball Heroes versions which wouldn't be an answer for the canonical universe, but an answer for one of the Dragon Ball versions.