Well, it seems that the power curves in Kubo's plot are fairly progressive and regular the older they are, the more powerfull they prove themselves to be. ( Ichigo excluded because he is the main char)
Gin, Blind Guy and Aizen are not (to me) uber since they lack experience compared to Yama-jii(Must point out that the Greezer is freakin uber since Ukitake &Shunsui were hardly relaxed when they had to face him.), Kenpachi, Ukitake, Kryouraku, Urahara.
I'm just saying that unless those three main evil Captains got some serious powerboost, or are fighting them 2 on 1 with the help of Evil Top5 Underdogs. They got no chance of winning.
If someone actually take down Genryussai it'll be utter bullshit since the old can just go Shikai and have everyone cooked in ten seconds. Don't even talk about having the old man going bankai because even if against Aizen or Top 1 Espada, it'll be like nuking a fly.
Note : Here again, it's just my personnal opinion, it's my point of view. I don't read Kubo's mind and I won't blame people with different vision of the thing. But excuse me when I say that Aizen being a betraying smartass son of a bitch with glasses doesn't make him the Strongest.