Alright, first, I think we should take a look at each group, consider the strongest few, and then compare them. It'll make it a lot easier and more fun.
So we start off with Ichigo's gang. To be fair, I think only Ichigo is powerful enough to be considered. Sure the others have been training, but it just doesn't compare anymore. For abilities alone, I'd have considered Orihime's space-time reversal, but she only uses it for healing.
Of Soul Society, only the Captains can really be taken into consideration. Of them, Yamomoto is the strongest, but his 3 disciples, Ukitake, Shunsui and Unohana, can also be considered. After that, I probably wouldn't consider anyone.
Then comes Aizen's gang. Aizen himself is obvioiusly the most powerful. While I think Gin is powerful, I don't think he's the kind who would win a one on one fight with one of the higher Espadas, for example. Speaking of, I'd consider Starrk, Barragan and Ulquiorra. Hallibel is ranked under Starrk and Barragan, but I think Ulquiorra's Segunda Etapa pushes him above her.
Of the Vaizards, only Hachi and the former-Captains can be considered (Shinji, Kensei, Love and Rose). I'm only including Shinji as we know what he can do.
I won't talk about XCution for the moment. We just don't know enough about them, though I'll throw in Tsukishima because, in theory, in a one-on-one fight, he can cut you and make him your friend, then literally stab you in the back.
Of Urahara's group... Urahara, Isshin, Tessai, Yoruichi. I'm tempted to throw in Ryuuken, but something tells me he can't really handle the power the others have here.
I'll do this in a weird way where I'll start from the list and slot them below/above someone:
I'll start with Ichigo. He is "neutral". [Ichigo]
Yamamoto obviously goes above him. [Yamamoto, Ichigo]
I'd probably rank his disciples as Unohana, Shunsui and Ukitake. Shunsui is scared of Unohana, and Ukitake is ill, but because of that, he can't stand up to Ichigo full-out. [Yamamoto, Unohana, Shunsui, Ichigo, Ukitake]
Aizen goes above everyone due to immortality. [Aizen, Yamamoto, Unohana, Shunsui, Ichigo, Ukitake]
Barragan's Respira is literally almost invincible. He could, in theory, make Yamamoto's flames die out (as all flames require a fuel. Even if Yamamoto's reiatsu is the fuel, reiatsu can fade as shown by Respira destroying kidou). [Aizen, Barragan, Yamamoto, Unohana, Shunsui, Ichigo, Ukitake]
Starrk obviously goes below Shunsui (but I think he could beat Yamamoto, and maaaaybe Ichigo on a stretch). Ulquiorra on Segunda Etapa is obviously lower than Ichigo, but higher than Ukitake. [Aizen, Barragan, Yamamoto, Unohana, Shunsui, Starrk, Ichigo, Ulquiorra, Ukitake]
Shinji... Is tricky. I'd probably throw him betweeen Shunsui and Starrk, because Shunsui is smart, and his powers are so... weird. But I think he can defeat Starrk at a push. [Aizen, Barragan, Yamamoto, Unohana, Shunsui, Shinji, Starrk, Ichigo, Ulquiorra, Ukitake]
Tsukishima... The thing is, if he cuts you, you're dead. So it's a case of "don't get hit". I think... probably everyone above Ichigo can avoid contact, though again, we've never seen him fight properly with anyone except maybe Ginjou. So I'll place him above Ichigo, but that's it. [Aizen, Barragan, Yamamoto, Unohana, Shunsui, Shinji, Starrk, Tsukishima, Ichigo, Ulquiorra, Ukitake]
Urahara, Tessai, Isshin and Yoruichi. We haven't really seen any of them fight, but I'd probably slot them around the level of Unohana and Shunsui. The only one I can say for certain is Urahara under Aizen. Because Aizen is immortal so he can never die, but he can still be "defeated" as we've seen. I'm sure he can find a way around Barragan, like Hachi did. [Aizen, Urahara, Barragan, Yamamoto, Unohana, Shunsui, Shinji, Starrk, Tsukishima, Ichigo, Ulquiorra, Ukitake]
Yoruichi/Tessai/Isshin would be TOO much of a guess. So here's what I have:
1. Aizen
2. Urahara
3. Barragan
4. Yamamoto
5. Unohana
6. Shunsui
7. Shinji
8. Starrk
9. Tsukishima
10. Ichigo
(11. Ulquiorra 12.Ukitake).
Though some places are obviously a lot more open to debate, such as Starrk and Tsukishima.