I'm still of the opinion that Gin will kill Aizen, in the end. I really doubt Aizen is the main villain. He wants to be the king of heaven, but who has the holy zanpakuto? Whose zanpakuto is nothing more than an illusion? Gin frequently taunts Aizen. Rukia hated Gin and said he made her feel sick. She thought nothing of Aizen.
Considering that, Urahara might possibly defeat Aizen. That would be the best way to end Aizen, imo.
Spoiler: We also have to consider that the people are fighting Aizen in waves of increasing strength. The fact that they left Urahara to the last after Yama-jin, Ichigo's father and Shenji, speaks volumes.