Jiren has more power and strength. Buu is kinda just evil and somewhat rage. He'll get beat up at first by Jiren but if he isn't terminated, he can eventually pass him. Jiren will likely terminate him before things escalate to that point.
Then came Frieza, Cell, Majin Buu, and many more, each one surpassing the other in strength. However, none of them come close to Jiren's strength.
Especially after receiving Super Saiyan Blue during Dragon Ball Super's Future Trunks arc, Vegito could undoubtedly beat Jiren with ease. A Fused individual's power is greater than the sum of their parts, and Vegito would also theoretically be able to access Ultra Instinct.
But it is true that Buu was at one point in time, far stronger than Beerus, to the point that Beerus could never hope to stop him. First off, let's look into some of what we know. Buu was the strongest being in the universe as stated by many prior to Super.
Even with his Super Saiyan 3 power, Goku already possessed enough strength to defeat both Majin Buu. His Super Saiyan God transformation would be more than enough to reduce even Kid Buu to a lifeless pink blob.
Kid Buu may not be as strong as Super Buu, but he's definitely strong enough to defeat Frieza. We know this because he's actually taken down Goku himself before.
Vegeta doesn't stand a chance against Kid Buu. The only verison of Vegeta that can defeat Kid Buu is the one that is in the final episode of Dragon Ball Z. Or if during the Majin Buu Saga he learned how to transform into a Super Saiyan 3.
5 MAJIN BUU: CAN SURPASS
He is able to continuously grow and can even train to further his power as seen in Dragon Ball Super when he became skinny Buu and went head-to-head in a sparring match with Goku. Goku has constantly acknowledged Majin Buu's limitless power and considers him a formidable foe and ally.
Buu is a brain dead troll with a lot of power, Frieza was more evil. Kid Buu was more like a mindless beast, he was evil, but he was more chaotic evil and wasn't at all Intelligent. Frieza on the other hand, outright enjoys to cause suffering.
Buu's power is such that, even without using his full strength, he was easily capable of holding out against Mystic Gohan, Super Saiyan Bra and a weakened Super Vegetto, and winning. His first real fight was with Gotenks, and even with the new knowledge of fusion that he had given Trunks and Goten, he easily won.
Gas would utterly destroy Jiren. Goku grows so much stronger in just the Moro Arc and then even more so in the Granolah Arc, and Gas still is stronger than Goku, who long since passed Jiren.
Black Frieza
This upgrade allows him to eviscerate the Heeter Force's Gas, but he also knocks out Ultra Ego Vegeta and Ultra Instinct Goku in a single blow. Jiren can't contend with either Perfected Ultra Instinct or Ultra Ego, so there's no way he'd stand a chance against Black Frieza.
What makes Jiren so strong in Dragon Ball Super? He's able to defeat a God of destruction and is even stronger than Beerus.
However, despite Ultra Instinct Goku nearly blowing Jiren clear out of the bottom of the ring, the Pride Trooper from Universe 11 won't concede defeat. In fact, Goku's speedy surprise attack tactics only make Jiren that much more furious, as he declares that, "Attacks that exploit openings are not true strength!"
Majin Buu proves to be powerful enough to defeat the absolute best of the best in battle, including most of the Z Fighters. He annihilates everyone on the Lookout, defeats Super Saiyan 3 Gotenks, and overcomes a souped-up Mystic Gohan.
Jiren is stronger than SSB Kaioken x20 Goku, he's also stronger than Ultra Instinct Goku who, if the lyrics of the episode 109–110 special (Goku vs Jiren) are anything to go by, is “100 fold” stronger than SSB. So no, Goku Black even as Super Saiyan Rose, doesn't stand a chance against him.
Zamasu isnt a good martial artist like kid buu. He doesn have a large skill set either. But with his immortality he would either eternally keep the fight going (because he couldn't kill kid buu, or would eventually win purely because kid buu is only near immortal while zamasu IS immortal.)
Out of all the villains in Dragon Ball Super, Zamasu is unquestionably the evilest of the bunch. His hatred for mortals grew to such a dangerous extent that he tried to end the life of every regular person with his draconic plan called Project Zero Mortals.
Ultimate Buu
Buu's “ultimate” form represents his most dangerous fusion yet. While fused with Piccolo, Buu manages to achieve his goal of absorbing Gohan. At this point in the series, Gohan is one of the most powerful fighters on Earth, meaning that Ultimate Buu wields an unprecedented level of power.
Zeno is omnipotent. Buu would absorb him, changing forms to more resemble Zeno. Then Zeno's consciousness would eventually take over Buu's body.
But Buu does have a regeneration limit, he could regenerate from being reduced to vapor by Gotenks, and the only way to beat him is to destroy him so completely that he is reduced to particles/atoms. Generally, regeneration is controlled by stamina and energy.
Someone significantly stronger than Boo, or who's on his level and can amplify their ki enough, could wipe him out completely with a powerful blast. Nothing short of total eradication can kill him for good, but someone with enough of a power edge on him could do that easily.
Buu eventually awakens and is taken to New Namek to confront Moro. During the fight, the Grand Supreme Kai takes control of Buu's body and it's revealed that Buu once had god ki, but it was destroyed when the pure Majin Buu was killed by Goku.
Super Saiyan 3 Goku was slightly stronger than Kid Buu. He even mentions to Vegeta that he COULD have defeated Kid Buu had he gone all out, but he held back to give Vegeta a chance to fight him. But he overestimated his living body's capability to sustain Super Saiyan 3, which was why things went so bad.
Ultra Instinct even offers more defensive viability and success than Ultra Ego. This just highlights Ultra Ego Vegeta's inability to defeat Jiren right now. Be sure to follow for more anime updates and manga news as 2023 progresses.