Power Tiers
Top Tier
Royal Guard- These shinigami are previously promoted captains of the Gotei 13. Their skills have been graded worthy of being the Spirit King's personal bodyguards. These shinigami are exponentially above average captain material, and possess some of the most fearsome skills among soul society.
Central 46 Six Judges- These judges are the strongest of shinigami in spiritual pressure, easily rivaling that of yamamoto in age and strength. In other words, there is no shinigami captain that will start out with a level of spiritual pressure that is comparable to yamamoto unless they are one of the Six Judges of Central 46.
Captains: The Captains are the strongest of all the Shinigami, all but one having Bankai and knowledge of kidou. A Captain can defeat all but the strongest Privaron and Espada without using Bankai. Stronger Espada are very difficult for most Captains to defeat.
The Captains fall into two general ranks; stronger Captains by reiatsu, and weaker Captains by reiatsu. The stronger canon Captains are Yamamoto, Shunsui, Ukitake, Unohana and Kenpachi. The weaker Captains are Soifon, Mayuri, Komamura and Hitsugaya. Byakuya falls into neither category and is an excellent example of Average Captain.
Vizards: Vizards are potentially twice as strong as an average Captain, with knowledge of Bankai and the ability to wear a mask that increases their strength by two, giving them access to hollow abilities.
Las Cincos Dios- These arrancar are the most fearsome hollow of Hueco Mundo. They possess a strength on such a level that they have been deemed worthy of being chosen as the Hollow King's elite personal guardsman as well as having direct authority over the ten espada.
Espada: Espada are the strongest Arrancar. The weaker levels, all numbers below four, can be fought by Captains using Bankai. The higher levels, Ulquiorra, Halibel, Stark and Barragan, are all beyond the ability of most Captains and can only be comfortably faced in single combat by Vizards.
Mid Tier
Privaron Espada:The Privaron are all ex-Espada. They outclass Numeros and their higher ranks are capable of fighting Captain level shinigami, such as Ichigo, on even ground.
Vice Captains: Vice Captains are second only to the Captains in Soul Society, and can, at their full strength, match Numeros level arrancar. Thus far, the strongest of the canon Vice Captains appear to be Renji, Shuuhei, Kira and Matsumoto.
Numeros: Numeros are arrancar Gillian's, for the greater part, and as a result they outclass seated level shinigami. The exceptions to this are Ikkaku and Yumichika, who are both at Vice Captain level despite being seated officers.
Low Tier
Menos (Discounting Vasto Lorde): A Menos Adjuchas can reach levels displayed by the strongest of the Vice Captains and even exceed them, however, for the greater part, an Adjuchas can be defeated by a Vice Captain, and Menos Grande by Higher seated level shinigami.
Seated Shinigami: These outclass normal hollows and shinigami considerably, particularly at higher seats. However, their strength pales against Vice Captains.
HIERRO
Hierro is the iron skin of an arrancar. It acts to protect an arrancar against attacks from other weapons or abilities. As an arrancar grows stronger, Hierro also grows stronger. Here is a breakdown of the strength of different hierro:
Numeros Hierro: Should be able to block the sealed sword of Seated Shinigami and low level Kidou without incantation from Seated Shinigami. Any seat above 5th with Shikai should be able to cut the hierro, as should any Kidou in mid range without incantation, or any kidou with full incantation. Any Vice Captain level Shinigami in terms of reiatsu should be able to cut a Numeros with a sealed sword.
Remember, however, that VC and Captain level Shinigami are at 1/5 full power when on Earth with their limiters on. This means a VC will normally be unable to fight a Numeros on Earth unless their limiter is released, due to Hierro.
Privaron Hierro: At lower levels, this can stop sealed swords from Vice Captains and at higher levels it can stop their shikai swords also. Most kidou performable by Vice Captains are ineffective against stronger Privaron.
Despite Ichigo's inability to cut Privaron's with Zangetsu, all Captains are able to do so with sealed swords, due to greater ability in reiatsu control, except Hitsugaya, who may require Bankai against the stronger Privarons.
Lower Level Espada #5-10 Hierro: Weaker Captains will have difficulty in cutting the hierro of such Espada with sealed swords, and mid/high level kidou is required to do damage from Captains. However, most shikai will be able to do damage.
Higher Level Espada #4-1 Hierro: The lowest of these Espada has displayed hierro capable of ignoring high level energy attacks. High reiatsu is needed to overcome these hierro.
As a rule, the following Canon Characters should be able to cut these Espada: Yamamoto, Shunsui, Ukitake, Unohana, Kenpachi, and the Vizard (masks on).
This is due to the fact that Kenpachi was able to cut Noitora, who had the strongest of all hierro.
KIDOU
Kidou is tricky and cant really be split the same way as other things. It can be used either with incantation of without. Using Kidou without incantation will usually reduce the effectiveness and requires more effort. As a general rule,
Captains: Can use high level Bakudo (binding art) without incantation easily and without power loss. Can use mid level Hadou (way of destruction) without incantation easily and without power loss. Some Captains are better at Kidou than others, such as Byakuya.
Vice Captains and High Seated Shinigami: Will have significant power loss in Bakudo and Hadou without incantation.
Low Seated Shinigami: Will have difficulty in using Kidou without incantation.
The power of Hadou (destructive arts) can then be split again;
67-99: When used by Captains, this will cause high damage to any enemy, especially with incantation at higher levels. For example, Hadou 90 at 1/3 of its full power can defeat a weaker Captain in a single shot. For Vice Captains and high seated members, these cannot be used without incantation, and are much weaker. Low seated shinigami cannot use these at all.
However, from non-Captains, these arts can hurt up to lower level Espada. From Captains, they have potential to maim and defeat the strongest foes.
33-66: Not quite as powerful as the high tier hadou, these still pack enough power to deal damage to Numeros and Privaron with incantation from non-Captains. Captains can use these arts without incantation and maintain full power. The higher level spells in this category can cause real damage to Espada opponents, when used at full power.
1-32: At lower levels, these hadou are mostly ineffective against the hierro of arrancar, with or without incantation. They remain useful against shinigami, however. Higher levels of these arts can deal damage at anything up to low Espada level (albeit low damage to Espada), however, this requires full incantation from a Vice Captain level shinigami, or no incantation from Captains. An example of the effectiveness is Hadou 31 from Abarai Renji against Szayel Aporro: minor damage was dealt despite full incantation.
