Kakashi Hatake was introduced as the leader of Team 7, consisting of Naruto Uzumaki, Sasuke Uchiha, and Sakura Haruno. Of his three students, Kakashi clearly favored Naruto and Sasuke, but which one was your favourite?
Perhaps Kakashi’s favorite student has always been Sasuke, and there are many reasons to believe so.
Just like Kakashi, Sasuke witnessed the death of all his loved ones, had the Sharingan and the same chakra nature. Sasuke was also consumed by the hatred of loss, which was something Naruto only realized after Jiraiya’s death. In that sense, Sasuke demanded more attention from Kakashi than Naruto did.
That’s why Kakashi decided to train Sasuke for the Chunin exams.
It didn’t expand just because Sasuke left Konoha, causing Kakashi to turn his attention to Naruto.
So given that Sasuke was easier to get along with, and that Kakashi could teach him more than Naruto, it makes sense that Sasuke was Kakashi’s favorite student.