I have 2 "Skyrim Besties". One is Paathurnax for 5 reasons:
1) His voice.
2) You can meditate on the Words of Power, making your shouts to have an extra effect.
3) He is the master Yoda of Skyrim.
4) The Blades can fuck themselves.
5) He is amazing.
The other is Kharjo. There is no best companion like him