Kenpachi would made a sandwich from Ichigo without Kubo's main character syndrome at their first fight.
Zangetsu gave all his power to Ichigo. If I correct, Zangetsu=Hollow Ichigo+His zanpakto(or how to spell it). He was already as strong as in hollow form. I have read a theory he was as strong as in full hollow form.
And Kenpachi told that when he fought Noitora that he so long had a good fight that already forgot how to use his muscle perfectly.
I think, Hollow mask Ichigo<Kenpachi.
Well, ichigo won fair and square, no arguing that.
Zangetsu =/= hollow ichigo. They are same, but sort of different. The one which is stronger appeared in his mind generally. (I don't mid spellings unless it's too pathetic. But I get you meaning well here :) )
1.Ichigo beat ken.
2. Catwoman told ichigo he couldn't beat kuchiki. So ichigo trained and fought, nearly on par, with him.
3. Hollow ichigo owned kuchiki in 5 strikes at most.
More importantly, ichigo's speed increased drastically after ban kai, so did power of his Attacks (GT, for example) Hollow ichigo's strength increased much greater than ichigo's. I doubt ken could keep up with him if byakuya was having difficulty..
Ichigo shouldn't have beat nor Ikkaku, nor Kenpachi, nor Byakuya... ok, maybe Byakuya. That was a CIS, against Ikkaku PIS. Byakuya wanted to kill Ichigo, he is master of the Kido's and IIRC only one times used it.
Ikkaku has far greater fight experience, he is one of the best swordsman. Ichigo has a little more reiatsu, but at that time he wasn't that fast or too powerful. Ikkaku's stick magically broke. Kenpachi has more experience too, but he didn't really want to win, he just enjoyed the fight.
Ichigo was get his zanpakto's all reiatsu, he told him. When he puts on his hollow mask he doesn't get his all reiatsu boost, only in full hollow form. And that is stronger than the bankai+hollow mask. So yeah, I think Kenpachi should be at least on Ulqiora's(true form) level.
Also Grimmjow has no chance against Noitora. The rank level has a system. Either the lower number the bigger reiatsu or just means how dangerous are they, but Noitora is "better" than Grimmjow, officially.