To be honest, I would either have to choose between Kurama or Gyuuki just on the basis that we only get to form bonds with these two beasts. In reality though I would definitely have to choose Kurama because we get to form a deeper relationship with him and watch his and Narutos bond grow from the very beginning. Kurama is a beast filled with hatred and is sick and tired of being stuck in a prison and throughout the plot Naruto begins to recognize that and sets out on a plan to take all of his hate away. Once Kurama and Naruto recognize each other for who they are they become a badass duo. Kurama was just a beast who was looking for somebody who could understand how he felt and I belive that everyone can relate to that at some point in their lives.
I just couldn't leave pictures of Gyuuki out. I mean he is a pretty awesome tailed beast too. Plus Killer Bee is hilarious!! And I love the storyline behind Killer Bee! Kurama is the most awesome tailed beast @tayhar18920