Iruka is #1!
The bond he forms with Iruka becomes the foundation for who he is as a person.
When he befriends Kurama and unlocks the seal to free him who does he think of?
Iruka.
When he's fighting Sasuke in part 1 and he's trying to understand Sasuke who does he think of?
Iruka.
When he's running off to war whose words does he literally take into himself?
Iruka.
Who does Bee say is at the center of all of Naruto's thoughts?
Iruka.
Who is the first to console Naruto after Jiraya's death?
Iruka.
Who puts his life on the line every time for Naruto?
Iruka.
Iruka is always shown as Naruto's first bond.
Without that bond Naruto would never have grown into the person he is.
Someone who reaches out to heal and to share.
When Naruto gives the whole alliance his chakra he is channeling his experience with Iruka.
It is true that Naruto didn't learn how to fight on the world stage from Iruka, he needed Jiraya, Pa, Kakashi and Bee for that.
However, Naruto could never have overcome his own hatred without the lesson from Iruka.
Naruto would not have taken Kakashi's lesson on teamwork to heart so readily if not for Iruka.
It should be said though that all of his sensei's and masters were of critical importance to his development into adulthood.
The fact that his first sensei was not a great warrior but a man filled with love, compassion and someone who could recognize that Naruto had already learned the greatest lesson by the time he was only 12 is really important.
That lesson: Naruto understood human suffering.
That leads Naruto to believe in friendship over force, humanity over nationalism, and the power of friendship to save anyone.
Without these core values what does Naruto become?
He would have killed Pain before talking to him.
He would have let Sasuke go.
He would never have conquered hatred.
He would never have understood the value of teamwork.
He would never have been able to help Obito.
He would never have been the boy to laugh with the tailed beasts and to sing their names.
He would never have been able to break the 1,000 year cycle of hatred.
Here's to you Iruka! The weakest and greatest sensei in the leaf village!