Welp, I guess I should have done this first before the big verse revision.
Bankai increases the combat ability of a Zanpakuto by 5 to 10x. Not all Bankai increase stats, some are increasing the effectiveness of a Shikai (like Yama's which increases the heat + army of skeletons for some reason, or Toshiro's which increases the amount of ice he can control), and others being completely hax based (Shinji's, Kyoraku's, Rojuro's, for example). IMO only the ones who showed an increase in stats should get it, this being the likes of Ichigo, Kenpachi, Ginjo, Komamura, etc.
The level of the multiplier is also a question because Yoruichi states it's between 5 to 10 times higher. So I propose 3 ends:
7.5 multiplier: 6 (WONDER3X, Bernkastelll, Secretly domain, Ghostzano, TOAAPRESENCE1, Meagboyo)
10x multiplier: 1 (DesiMaru777)
Resurrección was stated to have a power boost similar to that of a Bankai, so whatever multiplier we decide for Bankai would also apply to Resurreccion. But, I think this only should apply to the Resurreccion of the Espada, as the lower Arrancars had shown their Resurreccion being a smaller power increase than a Gentei Kaijo, and Aarroniero also stated that the Resurreccion of an Espada is different from that of a common Arrancar.
As shown during the fight between Ichigo and Grimmjow, Hollowfication is pretty much equal to Resurrección.
Vollstandig doesn't have a set multiplier, but it was stated to be much better than Letz Stil which made Uryu go from being somewhat comparable to a non-serious Mayuri to being able to one-shot Mayuri's Bankai (with an arrow 3x more powerful than his normal Letz Stil arrows). So I think maybe a 5x increase would work, but the same case as Bankai, only for those who showed an increase in stats as there are Vollstandig which are hax based (like Lille's, for example).
Agree: 8 (WONDER3X, Bernkastelll, The Pink God, Secretly domain, Ghostzano, TOAAPRESENCE1, Meagboyo, DesiMaru777)