Naruto views Sasuke as a brother, so he is more than a friend to him. I think Naruto would use the term "best friend" on Gaara and simply state that Sasuke is more than a friend, he is like a brother.
Watch sky high... his best friend became his girlfriend. His girlfriend became his enemy. His enemy became his best friend. God knows how the story will play. ..
Sasuke is Naruto's best friend by his choice ( Sasuke doesn't have any say )
but If i were in Naruto's shoes Gaara would be my best friend due to the amount of times he's helped me (Naruto)
I feel like the relationship between Sasuke and Naruto is too forced and therefore, I don't really consider them as best friends. As for the question, I would have to go with Kurama.