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 is no clear answer as to who would win in a fight between Itachi Uchiha and Obito Uchiha. Both are incredibly powerful ninja, and have unique abilities that would give them an edge in a battle. Itachi is known for his mastery of the Sharingan, while Obito possesses the power of the Rinnegan.
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 wins. Obito has his main MS ability to give Itachi a lot of trouble which would be Kamui and provides him the ability to be intangible for five straight minutes.
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 Uchiha was one of the strongest Uchiha to ever exist. He awakened his Mangekyō Sharingan during the Third Great Ninja War after witnessing Rin die. Obito's eye weilded the Kamui, a powerful space-time ninjutsu.
Obito didn't mean Itachi could beat him in a fight. He was scared Itachi might have set up a trap for him that could catch him unawares and end him.
After Sasuke, Madara is undoubtedly the strongest known Uchiha in the history of the series. 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.
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...
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.
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.
Obito wins. Obito knows his secret and his location. All he has to do is silently appear behind him and stab him with something or send him to kamui dimension and leave him there.
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.
As Obito had presented himself as Madara, and implied that he also bore hatred for the Uchiha, Itachi requested him to help wipe out the clan. However, Itachi also had a condition; Obito must spare Sasuke and the Leaf Village, to which he obliged.
Itachi would have surpassed Madara Uchiha if things were different for him. He learned every powerful Jutsu at a very young age and had a whole life to hone his skills and strength. However, he died too soon and his true potential never saw the light of day.
Zetsu was the weakest member of the Akatsuki. He specialized in infiltrating different places and gathering intel. Throughout his time in the organization, he was never involved in a serious fight that would show his fighting capabilities.
Who is the god of Uchiha? Indra Otsutsuki is revealed to be the creator/ founder of the renowned Uchiha Clan in Naruto. He was the eldest son of Hagoromo Otsutsuki (The Sage of Six Paths) and the elder brother of Ashura Otsutsuki.
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.
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.