Let me prephase this by getting the obvious out of the way; Zaraki gets stronger every time he come back from the brink of death.
Unohana：（そんな貴方にとって私は） (I was the first “adversary” you encountered) Unohana：（初めて遭遇する力の拮抗した“敵”だった） (Whose power rivaled your own)
Page 12 Unohana：（私は） (I) Unohana：（貴方よりも弱かった） (Was weaker than you)
Unohana：（生まれて初めて出会った“敵”と呼べるもの） (You thought that you would never again be able to enjoy battle) Unohana：（これを失えばもう二度と戦いを楽しめる時など来ないと考えた貴方は） (If you were to lose the first person you had ever met whom you could deem an “adversary”)
Page 13 Unohana：（弱い私に合わせる様に） (In order to accommodate my weakness) Unohana：（無意識の裡に少しずつ） (You unconsciously compressed your power) Unohana：（暗く深く） (And sealed it away, little by little) Unohana：（自らの力を圧し固め封じていったのです） (In a place deep and dark)
Unohana：（私は絶望しました） (I fell into despair) Unohana：（貴方に力を封じさせてしまった己の不甲斐無さに） (Despair at my own worthlessness for making you suppress your power)
Unohana：（しかしある時気付いたのです） (But then I noticed) Unohana：（貴方が） (Each time) Unohana：（強敵と戦い死線をかすめる度に） (You fought a powerful opponent and brushed up against death) Unohana：（少しずつ自らにかけた枷を砕き） (You broke the bindings you placed on yourself) Unohana：（昔の貴方に戻っている事に） (And little by little, returned to the way you used to be)
Unohana：（私は強い） (I am strong) Unohana：（貴方以外の誰よりも） (Stronger than everyone but you)
Page 14 Unohana：（だからこそ） (And that is why) Unohana：（私は貴方を殺しましょう） (I will kill you)
Unohana：（百度でも 千度でも） (Hundreds of times, thousands of times)
Unohana：（だからこそ） (And that is why) Unohana：（私は貴方を癒しましょう） (I will heal you)
Unohana：（何度でも何度でも） (Time and time again)
Unohana：（貴方が 真の貴方へと立ち戻るまで） (Until you return to your true self)
Page 15 Unohana：（そして） (And then)
Unohana：（私を超えて） (Surpass me) Unohana：（遥かな高みへ―――――） (And ascend to even greater heights-------)
In the beginning, he was introduced as a captain of the gotei 13 and the only one in its history to not have a bankai. It was at this time he could take virtually zero damage from Ichigo's shikai and was stated that this would apply to each and every Vice-Captain in the gotei who would try and hurt him.
All of this while he was still patched.
Ichigo pushes him to his limit and he is defeated and then taken for healing.
This all occured early in the SS arc, chapters 12-14 I beleive
Coming back Zaraki surprises two other captains by tanking their combined shikais, something they thought would kill even him with is repuation.
Zaraki goes on to defeat Kaname in bankai while not even removing his patch.
Again this is still SS arc
Zaraki next fights Nnoitra, the fifth espada with the strongest durability. He generally dominates Nnoitra until the later releases and at this point he can't win even with patch removed.
He uses kendo and one-shots him
Unohana heals him and later says this was an incident that brought him to the brink like Ichigo.
Kenpachi is able to take it to Yammy in R1 state (50% power).
Yammy is the most powerful Espada and stornger than Ulquiorra, Nnoitra, and grimmjow combined. To give a basic idea of a breakdown:
grimm (20) + Nnoitra (30) + Ulquiorra (40) =90
Yammy (100)>grimm (20) + Nnoitra (30) + Ulquiorra (40)
Zaraki goes from struggling with Nnoitra even without patch and two hands to defeating Yammy r1 with patch and one hand
Zaraki=Nnoitra=30, then he equals 60>R1 yammy 50 (Keeping in mind there was also a patch tax).
Zaraki's power should at least double
It could be more than double if Nnoitra makes up a smaller percentage of the three arrancar and/or the difference between Yammy and the three is bigger than 10%. However in the interest of not assigning arbitrary power ups without cause I've lowballed these numbers. At maximum, Nnoitra cannot make up more than 45% because Ulquiorra outranks him and even that that would leave Grimmjow with an exceedingly small PL by comparison.
All of this happens in the HM arc btw
After that there is a timeskip and the events of a novel occur if you beleive it to be canon. The basic gist as it applies to Zaraki is that he fights Cien and Soya. The first is an arrancar with the abilities of grants but a power greater than Yammy. The second is a former kenpachi who this site recognizes as island level for defeating the one before him, one said to be Royal Guard level. Assuming the events are true it should mean zaraki's power doubled twice or a total boost of 4x. Its worth noting that the novel introduced a new patch for zaraki because the old one wasn't doing the job anymore and so the new one absorbed ten times the reiatsu. Zaraki had the first patch since SS arc and since then he went through Ichigo (2x) Nnoitra (4x). The novel adds Cein (8x) and Soya (16x). so in total Kenpachi may be 16 times stornger so a new patch makes sense.
Zaraki stomps three sternritter in the first invasion without even removing his patch. He has a new eyepatch leading many to beleive the novel is canon.
He is then defeated by Royd(Or someone else, it is a moot point so I will say Royd)?
This should have doubled his power again (32x)
He then goes to fight Unohana who stomps him (64), over (128), and over (256), and over again (512, 1024, 2048, . . . .) Eventually he pushes unohana to use bankai and gets killed some more before finally surpassing her (refer to the beginning for a translation of the fight).
At this point his reiatsu can no longer be sensed, unless he voluntarily lowers to the degree of awareness putting him in good company with fellow SWP Aizen.
He also fights the self proclaimed storngest Strenritter and is largely winning even while holding back
Gremmy then doubles his power, so zaraki responds with shikai and we get a MC lv feat
Given the differnce between base and shikai is universally small in all other examples, and since we have a pretty cut and clear statistic handed to us on a silver plater via gremmy, we can safely say Shikai zaraki is 2x base Zaraki, which still puts him (Base) well within MC level (18.5 petratons tnt).
Also since Unohana raised his power by defeating all previous versions, and since the boost is 2x, we can put bankai Unohana somewhere between 50-100% base zaraki, still within MC level (9.5-18 petratons tnt). Arguably Unohana could be higher since the vast majority of fans across forums have long since said she would be exhausted after killing and healing Zaraki so many times.
So to sum up we have it written in black and white that:
Kenpachi gets a bit stronger everytime he brushes with death
Unohana's power rivals his and is stronger than everyone else, but is ultimately weaker then Zaraki
We have a clear calculation for Two Gremmy
By extention, we have a clear value for one gremmy, six gremmy, Shikai Zaraki, and base Zaraki, which translate to a value fro bankai Unohana
The only thing we don't know is how powerful base/shikai Unohana is. As a minimum >than post Royd Zaraki, but that's a bit like scaling Shunsui to chad via their SS arc fight (its accurate but also an extreme lowball). Another option is to look at other captains and the difference between their shika and bankai state and apply a similar value increase to her. Thirdly there is the bankai multiplier available, though it does tend to be ignored on this forum in favour of larger increases.
EDIT: MC will change to new value pending calculation group's new result.