I look at it like this.
God Tree/Chakra Fruit = God Tier like Level 20
Kayuga = Lvl 15
Sage of the six paths (Hagoromo as Juubi Jinchuuriki) Also Level 15 maybe higher once mastering his powers.
Hagoromo and Hamura Birth Power Level 13
Indra and Asura comes in around level 12
They are Parallel Characters like their transmigrants. Madara and Hashirama (Level 11s)
Naruto and Sasuke are level 10 once they hit theyre normal potential. Kyuubi Naruto and EMS Sasuke.
They jump to level 12 with Rinnegan and Senjutsu of So6p.
What we should focus on is Hagoromo and Hamura were able to defeat their mother and seal the juubi without any special powers. It was their birth strength. Therefore I believe Indra, Being the FIRST Sharingan user, The reason it exist, would have had to be extremely powerful. We also know he was a user of the Perfect Susanno.
Asura all we know is he had an avatar like Naruto, and is on Par with his brother. I believe noone would believe naruto could be on Par with sasuke without kyuubi but... Asura seems to be known for doing it. Hashirama had wood style.
Lately what's most important now is I believe sasuke could past Indra as he has Rinnegan. I'm pretty sure only a rinnegan user can see and manipulate all chakra like that. THat's why he could combine all the tailed beast, and steal naruto's like pain could. Naruto shoulda gave sasuke some sage chakra, wouldn't that be funny.
But Indra was bad ass. He got an arrow named after him.
EDIT: A comment to the comment above, Salamander, I believe Ashura just worked hard and was able to obtain the same power ass Indra.
Here's Some Indra Stuff
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and Asura
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