I don't think the spirit king is THAT strong because he needs so many royal guards to protect him, IMO, this is how I go with it:
Strongest Shinigami: Yamamoto
Strongest Vizard: Ichigo
Strongest Arrancar: Aizen
I think Aizen has become a new type of Arrancar. (Not an espada and not even a Vasta Lorde) The power he gained is something no one can even touch unless you have similar powers. If I want to rank them:
Ichigo / Yamamoto > Yamamoto / Ichigo > Aizen (without the hogyoku's powers)
If Aizen hadn't been so cheap to use WW, he would have lost to Yama's shikai alone... Next:
Aizen (with hogyoku's powers) > Ichigo / Yamamoto > Yamamoto / Ichigo
If Ichigo had more experience, He would have top Aizen man... Btw, if your wondering why I put Ichigo / Yamamoto> Yamamoto / Ichigo, it's because it depends on how the situation goes. Right now, Ichigo has all the powers to match Aizen even in his evolved form. However, if he does not learn how to control them, Yamamoto would pwn him with his bankai... (His Bankai would be like more fire? Thats deadly...) If Ichigo does learn how to control them, he could defeat Yamamoto, but not Aizen in his evolved form, Ichigo still needs something else to defeat Aizen in his evolved form.