There seems to be quite some confusion. Therefore, please pay attention to the following explanation.
To begin with, Hollows can be classified as regular or menos grande, the latter category is where all of the Espada fall.
The menos grande has three subdivisions in which Hollows can be placed, primarily off appearance and strength:
- the lowest level is known as the Gillian class
- the middle stage is known as the Adjuchas class
- the final level is known as the Vasto Lorde class
To move from 1 to 2 and 2 to 3, the Hollow in question must apparently consume other Hollows.
- that is what Grimmjow and his crew were attempting to do prior to becoming arrancar
Arrancar are entirely different though, any Hollow, regardless of their classification may become an arrancar.
Regardless of what class they were as Hollows, they can be naturally created or artificially created (Hōgyoku).
Even though both forms of evolution lead to viable arrancar, the latter method generally creates stronger members.
The above notion is directly correlated to the status of both the Privaron Espada (natural) and the Espada (artificial).
- the former, whilst conceived first, were placed in a lower tier than their more recent members
There is no such thing as a perfect arrancar, their power is based off previous Hollow category and method of evolution.