First, what exactly is a "Friend"
Second, you are not the characters...nor have you been raised in an environment similar to the characters/most likely. so why should they be restricted by what 'you' see as friendship?
for sasuke, Naruto is his best friend because he is the only one Sasuke acknowledged as his friend in the first place.
They have done more to be considered friends then most peoples real life friends...Saved each others life/Stuck up for someone even when they committed a crime, Sasuke admitting that he cared about his teammates during the Garra battle was enough...people just like to twist sh*t, so they'll say that means they were comrades not friends.
Sometimes people will say Naruto was dumb for not knowing who his 'real friends' were...simply because he use o skip class with a few people. (but they completely disregard those characters attitudes towards Naruto when they were first introduced)