- Are you are asking that who makes him want to be the Pirate King?
If this is the case, there are only few people that initially made him want to become the Pirate King:
Shanks, Ace, Sabo
( Great icons of his youth, Roger ofc too, but he hasn't met him )
The Straw Hats Pirates.
The people to whom Luffy has sworn that he'll be the Pirate King.
- Or are you asking who will be the "greatest" ally for Luffy?
I doubt that Shirahoshi will play a big part in future, it's not like she _wants_ to use her abilities as a weapon. And if they were to go to the war, it would undermine their progress to get closer to the surface dwellers.
The Straw Hat alliance will obviously play a big part, I honestly can't see any reason for Oda to mix more sides to future major battles (other than Pirates and those that really are involved), it would just get messy and cheesy.
"OH YOU CAME TOO, I'M SO GLAD!" (I can hear Luffy saying that in Jap) -> in anime, let's watch next 4 episodes as flashbacks from the past.