Itachi wasn't strong, he was super smart. Obito feared his smartness. He knew Itachi would destroy any plan of obito as long as he's alive. Obito was not afraid of dying, he was just afraid of his plan getting ruined.
Obito was not afraid of Itachi. He was only cautious of him. He himself said that “If Itachi knew about him…” but Itachi didn't even know that he wasn't Madara.
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.
Therefore Itachi's only choice was Obito, Itachi knew Obito (who he thought was Madara) wanted revenge against Konoha and the Uchiha clan and knew he could've taken on the Uchiha Police Force with minimal difficulty because he had Kamui (Which is a really difficult technique to counter).
Obito decided to "break his promise" about not attacking Konoha when Itachi died, which implies he wanted Itachi to die anyways.
Shisui's DNA Could Not Be Found
Considering Kabuto was able to acquire the DNA of Madara and several other shinobi that had passed away a long time ago, it makes no sense for Shisui's DNA to be unobtainable.
According to the wiki Obito was 31 years old in part 2, itachi was 21.
His nickname (Tobi) comes from Obito calling him Guruguru (グルグル), an onomatopoeia for turning round and round or going around in circles. It can also mean "wrapped around", referring to Tobi's function as armor for Obito.
For sure he knew he could kill him 1v1, and as he said in Akatsuki, if Itachi knew the truth about him, he would be dead.
Obito Uchiha was one of the main antagonists in the Naruto series who influenced the plot by a huge deal. Obito was in the Akatsuki and was known as Tobi. Not many people knew his true identity and his mask did a great job in concealing it. Only the Zetsu and Madara Uchiha knew about this shinobi.
Undoubtedly, Kaguya Otsutsuki was much stronger than Obito.
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.
10 Could: Itachi Uchiha
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.
While Itachi was not overly friendly with the Hatake prodigy, the Uchiha did respect Kakashi. They had a tentatively symbiotic pairing between them, but it all fell apart when Itachi stopped fearing Kakashi and viewed him as an obstacle to clear.
Obito feared his smartness. He knew Itachi would destroy any plan of obito as long as he's alive. Obito was not afraid of dying, he was just afraid of his plan getting ruined.
Itachi was not scared of Jiraiya, he just understood the level of difficulty he faced in that matchup. In Naruto battles were not always about who's-stronger-than-who, but rather who has the better matchup due to their skillset.
So Obito can transfer any of his body parts that are in contact with the enemy to the time-space of the Kamui and make it look like he cannot be touched.
Though, he can not transport pieces of himself to the other dimension, nor can he touch someone and send them there. Obito could transport himself, and pieces of himself to his dimension. But to transfer others, they have to be within touching range, or actually touching.
He chose Itachi to join the Akatsuki because he wanted to use him in case Nagato betray him to revive Madara back to life.
Obito's hate for the Uchiha was simply a result of Madara's animosity towards the clan, as they had betrayed him long ago and had decided to side with the Senju-led government.
Obito wore a variety of different masks over the years to hide his identity when he interacted with others.
When Obito first comes into contact with Madara, he refers to him as his precious forefather. Having Obito say that could mean that he is a direct descendant of the legendary ninja. It does explain the reasoning behind taking up Madara's namesake as his own after becoming leader of the Akatsuki.
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.
Obito played a significant role in the Fourth Great Ninja War. However, after redeeming his evil actions, Obito died aged 31.
Itachi is the brother of one of the main characters of Naruto, Sasuke. For much of the series's duration, Itachi was reviled by fans for killing his and Sasuke's entire clan, the Uchiha clan.