In which episode does Kakashi think Naruto is his sensei?
The episode is #329 of Naruto Shippūden, titled "Two-Man Team". It's manga equivalent is Chapter 571, "Tailed Beast Mode!!". The scene you're referring to is at 08:05.

Kakashi mistakes Naruto to be his former sensei right after Naruto deflects the five Tailed Beast Balls with astonishing speed.

Here's the scene from the manga: