All those FF references go over my head, but comparing Aizen and Ichigo so far means little. Ichigo might be overly powerful considering he should still be very green but at least he has a huge weakness while Aizen seamingly has none.

Of course I don't see the end yet, and don't even read the manga, but normally one would assume Aizen collected his buddies and then developed the Arrancar to cover his own weakness (whether that weakness was in his defense or required to execute his plan). Now it seems like he had no use for anybody and they just were there because... the mangaka decided the story needed more fights.