This estimation is a huge generalization and ignores individual match ups like Luffy vs Enel for example, it is obvious that there are exceptions but this is how i see it right now:
Vice Admirals = Crocodile pre-TS, Jinbe(on land), Zoro, Sanji < Kidd, Law, Kuma (mindless), Luffy, Ace < Vice Captains and commanders of Yonkou (Marco, Jozu) < Doflamingo, Hancock < Mihawk = Admirals, Garp, Rayleigh =/< Yonkou
Explanation:
Vice Admirals´fighting style has been shown to consist of this: Rokushiki + Haki + either a weapon or a DF (Onigumo has both). Of course they belong to the strongest fighters in the OP world but it still is way inferior to the real big players. I think Sanji and Zoro are pretty much on their level now, nothing that the VAs have shown exceeds the feats both Zoro and Sanji have shown. Some people might say that this estimation overestimates the SHs (including Luffy) but looking at the opponents they are soon going to face, i think it is appropriate. Crocodile pre-TS has one of the deadliest fruits we have seen, his Logia gives him an advantage in movement and in variety of his attacks but what he lacks is Haki and advanced physical abilities. Jinbe is pretty much the opposite, being a strong physical fighter but without a deadly ability.
Of course there are weaker and stronger characters in this level while the lines between the levels are also not absolute, especially considering that there are no real tiers in OP because of the variety of different abilities but this is just a simplification and generalization.
Next level is a bit stronger than the one before. Law has one of the strongest abilities in OP, he almost becomes god in his room but what he definitely lacks is physical abilities, meaning strength, speed and endurance which also reveals his biggest weakness, namely melee. Smoker, though being weaker than Law overall, was able to corner Law for a certain amount of time and even almost got him. Kuma also has one of the strongest abilities in OP, is very fast and physically strong (probably) but since his transformation, he lacks two things, namely intelligence and endurance. The former should be clear, the latter is because of him being a Pacifista, either you are strong enough to break or cut the the exterior of a Pacisfista or not. Kidd is on this level solely because of his portrayal as rival of Law and Luffy. Luffy has impressive physical abilities (speed, strength and endurance), is one of the few who has all 3 types of Haki, even if its not advanced enough yet, has immense destructive power (Noah, Kraken) and since the timeskip, a deadly attack, namely Red Hawk. These points already make him very strong while still giving him room to grow.
Next "level" is the vice captains and strongest commanders of a Yonkou. They are capable to fight Admirals for a longer time, have strong and unusual abilities, impressive physical skills and a high level of Haki. From what we have seen (could obviously change with the other commanders) is that they lack deadly abilities. Marco purely fights on a physical basis while Jozu turns into diamond and still has to punch the opponent.
Next level is extremely close to the admirals but not completely equal, they are capable of beating the admirals under certain circumstances though. Both Hancock and Doflamingo have all 3 types of Haki, have shown impressive physical feats and most importantly, both have extremely deadly abilities that are also extremely hard to defend against. Hancock is generally underestimated and underrated, probably because she has not been portrayed and hyped as a villain and antagonist.
Next level is the strongest one where the biggest players play. Mihawk´s estimation is pretty much only based on the symbolism, namely him being the rival of Shanks while the latter was able to clash with WB. Admirals were able to clash with WB, Rayleigh was able to clash with Kizaru, Garp is a legend and the only one who could fight WB and Roger equally in their respective prime, the Yonkou were in a stalemate.
