All Hollows begin normal and to achieve the next rank must devour souls.(human, shinigami, even other hollow)
rank 2.Gillian-must eat 4 human souls or 3 shinigami souls or 4 hollow souls
rank 3.Adjuchas-must eat 6 human souls(in addition to the souls already eaten at the previous lv) or 4 shinigami or 5 hollow
rank 4.Vasto Lorde- must eat 8 human souls or 6 shinigami or 7 hollow
rank 5.Arrancar-achieved when the Hougyoku has been used on the vasto lorde to remove its mask.the 10 strongest belong to a group called the Espada