Conclusion:
So assuming Goku regains his tail at some point, I think in my DB fan fic mind, Goku's peak will be.. SS4 UI/ True Saiyan Instinct/or just MONKEY GOD. Fitting for a Sun Wukong. A monkey that ascended to the heavens.
(14/14)

The only issues are the panels at the beginning of DBS. I don't know how to explain this other than a retcon or there being very specific conditions like Goku having his tail--hence the "right now" this is my final form. Emphasis on "right now"
(13/14)

So, what is Goku really? We all know... He is a Saiyan, or as Freeza loves to call him, a MONKEY.
(12/14)

Could this be a reference to God Ki? Perhaps Daima's red hair SS4 is alluding to Goku's prodigal potential and proximity to God level Ki, which may further reference how Goku was able to absorb the SSG ritual into his base form in Super. (11/14)

Unlike GT where SS4 hair is black and fur is red, SS4 in Daima has red hair and red fur... But then there is Limit Breaker SS4 from SDBH (NOT diving into SDBH)
(10/14)

Interestingly, it's a Namekiam in Super and Daima that connect Goku to his Saiyan roots: Neva and Monaito (9/14)

So, Daima has now made the ultimate Saiyan form canon. Neva unlocks Goku's latent Saiyan instinct/DNA causing him to regrow a tail--like GT, this results in Goku reaching the Saiyan Peak (8/14)

Although not canon, GT also sorta tackled this question by having Goku return to his Saiyan Roots--the regrowth of his tail and the Ozaru transformation. Goku literally had to reconcile with his inner voice to achieve SS4
(7/14)

Goku visits Whis again during the battle with Gas, and Whis again reminds him that as an angel he is a different life form from Goku and I don't believe this is just about Goku being a mortal... "WHAT IS GOKU"?
(6/14)

Fast forward: Goku remembers his father Bardock "Awakening" something within him+ remembering his Saiyan Pride.

This results in him using TUI in which he fights with the emotion of a Saiyan while using UI-- Developing an instinct unique to himself, a Saiyan. (5/14)

Two interesting things come from this:
1. Granola mentions the Ozaru form that terrorized his people
2. Vegeta's response to seeing UI + Super Saiyan form -where he states Goku's UI is more accurate when added to a SS form
(4/14)

Throughout the training portion of the Granolah Arc Goku struggles to grasp his path & starts to adapt the essentials of UI into his base form to conserve energy which is supported by Whis. However, in combat vs Granolah he starts to use his Super Saiyan forms+ UI (3/14)

Vegeta has to learn to let go of his Saiyan Guilt to freely use "Destruction", however, Goku is actually instructed NOT to copy Angels--but rather to find his "Own Unique" style. (2/14)

In the Granolah Arc Goku & Vegeta pursue their specific paths to power/ and in the post Broly arc, Gohan also finds his own path. Specifically Goku and Vegeta are following the Angel and GoD paths respectfully. (1/14)