In conclusion, Obito Uchiha would defeat Itachi Uchiha when comparing the two characters in their strongest forms. Obito has one of the strongest, if not the strongest, Sage Mode in the series and has enough chakra reserve to beat Itachi.
There are a few reasons why we might consider Obito to be stronger than Itachi. For one, Obito was able to defeat Madara Uchiha, who was considered to be the strongest Uchiha of all time. Additionally, Obito has access to the powerful Rinnegan, which gives him a wide range of abilities that Itachi doesn't have.
Sasuke has mastered the powers of the Uchiha and also unlocked Six Paths powers. He was powerful enough to go toe-to-toe against Madara Uchiha, meaning he's much stronger than Obito.
Obito was a late bloomer in the beginning, but he continued to excel and become a formidable foe against his enemies. However, Obito can't defeat Sasuke, as the latter, after receiving chakra from the Sage of Six Paths, became stronger than him and was able to fight against the likes of Madara and Kaguya.
Itachi, while strong, would've been no match for Obito in his Ten-Tails state. The legend of the Uchiha clan, Madara, is another Uchiha deemed to be far stronger than Itachi. Madara lived during the Warring States Era and his powers were honed to match Hashirama Senju in battle.
Sasuke Uchiha
In his case, he surpassed Itachi during the 4th Great Ninja War, where he gained the power of the Rinnegan, along with the Six Paths Yin chakra. Consequently, Sasuke became one of the strongest characters in the entire series, with Naruto being the only human capable of matching him.
Kakashi does battle Itachi early in the series. Itachi ends up trapping him in a genjutsu and draining Kakashi. The contest basically ends up in a loss for Kakashi, but that is somewhat debatable. Still, Itachi shows more control over his Sharingan and the ability to match Kakashi.
Obito's Ability To Phase Through Anything Would Be Too Difficult For Jiraiya To Handle. Obito was able to phase through anything, possessing reflexes to obviate Minato. This is profoundly impressive considering that the Fourth Hokage was considered by many to be the fastest ninja to have ever lived.
8 Obito Uchiha, The Second Ten-Tails Jinchuriki
The First Hokage, Hashirama Senju, openly admitted to being inferior to him. Given that the Five Kage are weaker than both Hashirama and Madara, they would certainly not have stood any chance against Obito either.
His mastery of chakra control and physical abilities allowed him to become a member of the Anbu Black Ops when he was 11 years old. Despite his impressive achievements, there is no way Itachi can beat Obito in his most potent form. Obito Uchiha, in his strongest form, was the Ten Tail Jinchuriki.
If Shisui was his sibling he would definitely know since they were both living with the Uchiha and Shisui should only be around 4-8 years older than Itachi who is 9 years younger than Obito, hence they should also know each other...
Itachi was able to fight against Naruto Uzumaki in his Nine-Tails Chakra Mode, something that Pain could never do. He was also able to incapacitate Nagato Uzumaki, and it is quite easy to see that he could definitely have taken on and defeated Pain in combat.
During his infiltration, he encounters Minato Namikaze (the fourth Hokage) and they fight. Minato defeats Obito, and releases Kurama from Obito's control. Obito then flees, leaving Konoha to deal with the Nine-Tails, resulting in Minato and Kushina's deaths.
Like Obito, he became the 10 Tails Jinchūriki in the Fourth Great Ninja War. Madara was much stronger than Obito to begin with, and the fact that his version of the Ten-tails was perfect made him even stronger. When compared to Madara, Obito stands absolutely no chance.
He was scared Itachi might have set up a trap for him that could catch him unawares and end him. He said that right after this happened.
Considering how badly Kakashi and Minato's attacks devastated him in separate duels, Obito would not survive being punched by Tsunade. However, his kamui allows him to become intangible at any time, rendering it nearly impossible for his opponent to land a hit.
His powers far surpassed the likes of the Five Kage, and even the old Hokage combined. Obito was truly an incredibly powerful fighter with few being strong enough to hold their own against him. He was far stronger than Pain and his ambition was likely much greater as well.
4 Triumphant Victory: Kakashi Vs Obito
During his fight with Kakashi in the Kamui Dimension, the two ninja's seemed surprisingly equal. While Kakashi does come out on top during this battle, it is partly because Obito needed Kakashi's Lightning Blade to hit him to break Madara's control over him.
Obito is stronger than Kakashi. Why did this dull head lose the fight in the dimension? I'll take it that you're talking about the fight in Obito's dimension. It looked like Obito lost, but he didn't.
With the power of these two types of Sage Mode, Naruto was able to beat many strong enemies, including Madara Uchiha, Obito Uchiha, and Kaguya Otsutsuki. Towards the end of the series, Naruto could comfortably fight and defeat the Five Kage on his own with this power.
Although Minato defeated Obito once when he was alive, if the latter fought the former in his Jinchuriki Transformation, he could defeat him with ease.
Naruto and Sasuke are arguably the most powerful characters by the end of the Naruto Shippuden anime, so it goes without saying that Might Guy would stand no chance against Naruto in his Sage of Six Paths mode, or against Sasuke using his Rinnegan.
Sasuke would have only won by outlasting Itachi, and if the latter had no reason to hold back then he would have gone all-out. When the two brothers fought against Kabuto, it was only because of Itachi's knowledge that they managed to defeat him.
Sasuke is yet another strong character in the Naruto series who eventually surpassed Kakashi. Sasuke was so strong that he was able to perform jutsus like Indra's Arrow which has similar, if not the same destructive capabilities compared to Naruto's Six Paths: Ultra Big Ball Rasenshuriken.