Round 1 Base Aizen
Round 2 Aizen 2nd form
Round 3 3rd form Aizen
Round 4 Butterfly Aizen
Round 5 Monster Aizen
Well the Akumaito Kosen (aka the Devilmite Beam) would definitely come in handy, since Aizen is very evil, so that's one thing in Akkuman's favor.
I will say more later though.
Edit: After reading more about the Akumaito Kosen, I have realized that it will be an instant kill if Akkuman can use it against Aizen, since it literally causes the victim's heart to explode if it works on then, so Akkuman can win unless he is outsped, which Aizen is very fast.
Also, is Aizen going to be allowed to regenerate, or not? Because then he wins if he can, but if not, again, it's a matter of who's faster.