Even if the Quincy final boss the final boss of the manga, he just doesn't have the cool, nonchalant, effortless, handsome appeal as Aizen did. Aizen was simply awesome because he was the biggest troll and there must have been 8 or 9 times during that arc where everyone thought they killed him but didn't.
Have to admit this deserves to be in the top 10 coolest anime moments in history.
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I also wonder if Unohana, Yorichi, Urahara, Kyoraku and Ukitake were holding back against Aizen because none of them really fought Aizen directly or even used their Bankai.
http://tvtropes.org/pmwiki/pmwiki.php/Main/DiscOneFinalBoss
The fact that everyone got a powerup lately and could now probably one-shot Aizen if they fought him again kind of devalues Aizen and Captain Commander Yamamoto. All these squad0 guys and the powers they gave to everyone, it's like why weren't they there to kill Aizen? It's reminiscent to DBZ. Frieza was the defacto strongest in the universe yet where was Baby, Majin Buu, Brolly, Cooler and basically everyone after Frieza who had like 10000x higher powerlevels than him?