Majin Buu stated he was Omnipotent , but since we clearly can't highball such statements to tier 1 or 0, we must take the alternate option:
Low 2-C Buu Confirmed (4D is infinite to us 3D beings, plus, he was able to rip apart dimensions)
Gohan was able to overpower Buu in Mystic Gohan form rather casually and even blitz him.
2-C Gohan confirmed
You know who's stronger than Ultimate Gohan? Vegito. Casually at that. Also, two saiyans with limitless power. Limitless = Infinity, so now we have two infinites, one in infinite 4D power, and the other casually elevating them to 5D.
High 2-A Vegito confirmed.
By the time of Battle of Gods, they have vastly surpassed their previous peak in power. "Super Saiyan God" is obviously infinitely above such forms such as "Vegito". Let's just face it, it's limitless even to the likes of Vegito.
Low 1-C SSG Goku confirmed. TIER ONE DBZ HAS ARRIVED!!!
Super Saiyan God Super Saiyan multiplies that power by 50... let's see... 50 infinites? It makes SSG look like a joke in power, which makes sense in context. So we gotta go with this:
1-B SSGSS Goku confirmed. Vegeta also scales.
Golden Frieza was able to easily overpower Goku in that state and it took both of them to beat him. This means logically he is 100 infinites in power.
1-B Golden Frieza confirmed
Until they were even able to power up with their super omega Kaio Ken, Hit was completely overpowering them, making SSB Goku look like a joke in comparison. Of course, logically, this means that Hit is capable of higher power.
1-B Hit confirmed. Possibly far higher.
You know who was able to overpower Hit in their Kaio Ken x10 form? Goku. And you know what he did? He punched him, and even went faster than time itself! That takes like being an infinite degrees of infinity above time to do that! This could only mean ONE THING...
High 1-B SSBKKx10 Goku confirmed
So uh, where do we go from here? Of course, I know. We still have the God tiers to talk about. Take Champa for example. Champa is absolutely boundless even to the perspective of Goku, meaning that he is beyond such restrictions that Goku and lower characters are constrained by, making him unbound.
1-A Champa confirmed, time for some DBZ vs Cthulhu Mythos matches
One thing that's to note is that Beerus is canonically stronger than Champa.
1-A Beerus Confirmed
You know who made a complete joke of those 1-As with a single finger? Whis and Vados. They are so absurdly powerful they completely dwarf Beerus and Champa by an immense amount, so likely, we will have to make to do with this tier.
High 1-A Vados and High 1-A Whis confirmed!!!
The absolute strongest being in all of Dragon Ball, completely boundless even to the perspectives of Vados and Whis, are courtesy to Zeno, one of the absolute strongest in all of fiction; there are no restrictions, all of Dragon Ball bends to the existence of Akira Toriyama's Author self Insert: Zeno.
0 Zeno confirmed...
Oh, and when they mean 12 Universes, they mean 12 realities that are all infinite dimensional.