This is based on Volume 10, Chapter 83, Page 80 (page 14 of the chapter): Ganju said that when Kaien enrolled int the academy, Kaien's spirit-energy (reiryoku) were 6th class spirit-powers (roku tō rei'i) and equal to the previous 13th squad's vice-captain.

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霊力は六等霊威 | Reiryoku wa Roku Tō Rei’i: Spirit-Energy Class of Spirit-Powers

霊 | Rei: spirit/soul/ghost

力 | Ryoku: energy/power/force

等 | Tō: rank/grade/class

霊 | Rei: spirit/soul/ghost

威 | I: prowess/power/influence/potency

Also a little unrelated, but I personally think spirit-energy is stored at the soul-sleep, whicjh is like a pressure point (魄睡 | hakusui), and the chain-binder (鎖結 | saketsu) is another pressure point is where spirit-energy flows from, to the rest of the body. While spirit-pressure (霊圧 | reiatsu) is the force the spirit-energy exerts when the spirit-energy flows throughout the body. Wasn't exactly explained like this in the manga, bu that's how I see it.

Anyway, I think of things like this, based on just spirit-energy only (not other stats or unique abilities). I think you could interpret it the other way around with 1st and the strongest, and 10th as the weakest, but i think the this fits better personally. Because #6 should ideally be vice-captain, but if you invert it, vice-captain doesn't fit at #6 in.

• Unclassified: Human without enough spirit-energy to see souls
• 1st class: Human with enough spirit-energy to see souls
• 2nd class: Academy Student
• 3rd class: Shinigami with Asauchi / Unseated Officer Shinigami / Hollow
• 4th class: Shinigami with Zanpakutō / Low-Seated Officer Shinigami / Huge Hollow
• 5th class: Shinigami with Shikai / High-Seated Officer Shinigami / Menos-Grande Hollow
• 6th class: Vice-Captain Shinigami / Gillian-Menos Hollow / Non-Menos Arrancar
• 7th class: Shinigami with Bankai / Adjucha-Menos Hollow / Gilian-Menos Arrancar
• 8th class: Captain Shinigami / Vastolorde-Menos Hollow / Adjuchas-Menos Arrancar / Espada 5-10
• 9th class: Senior Captain Shinigami / Royal Guard Shinigami / Vastolorde-Menos Arrancar / Espada 1-4
• 10th class: Ichigo / Yamamoto / Ukitake / Zaraki / Aizen / Ichibē / Yammy / Yhwach
• Unclassified: Final Getsuga Tenshō Ichigo / Divine Aizen / Soul King

And then there are these guys I'm not sure about:

• 4th class: Fullbringers
• 5th class: Limit-Sealed Vice-Captain Shinigami / Fullbringers with Ichigo's Power
• 6th class: Limit-Sealed Shinigami with Bankai / Ginjō / Tsukishima
• 7th class: Limit-Sealed Captain Shinigami / Ginjō with Ichigo's Power
• 8th class: Ginjō with Bankai / Sternritter here?
• 9th class: Or Sternritter here? / Ulquiorra here?
• 10th class: Elite Sternritter here? / Or Ulquiorra here?