Who defeated Sosuke Aizen?

After waging war against Soul Society with an army of Arrancar, Aizen was defeated by Ichigo Kurosaki and sealed away by Kisuke Urahara, and then imprisoned for his crimes.

How did Ichigo defeat Aizen?

Ichigo slashes Aizen across the chest after destroying Hadō #90. Kurohitsugi. Pushed back a distance, Aizen collects himself and tells Ichigo to not talk so triumphantly because his physical strength momentarily exceeded his own.