I admit I'm not very familiar with Naraku, but I'm going to hand this to Aizen anyway. Aizen utilizes Kyoka Suigetsu and suddenly Naraku is thinking he's in a nice field of burning flowers while Aizen brutally butchers him. Or Aizen just plain beats the crap out of him. Fairly certain speed, strength, and durability are all a bit beyond Naraku's level. And the Espada could probably take down any of Naraku's minions with ease. I repeat: I don't know much about Naraku, so I'm open to suggestions. I'm fairly certain Aizen is just a bit too... well for lack of a better word, hax, to be beaten by Orochimaru's alcoholic father.