Aizen would win.
Sure, Yamamoto is strong. But Aizen is stronger. He even took down a captain with a destructive art #90 (black coffin)
So he could just fire one of those at Yamamoto and he would be beat or pretty heavily damaged. That or Aizen would just use the good old "Suprise, i'm actually standing behind you and not 30 yards away. Oh, and I stabbed you 4 times."
Aizen > Everyone in the manga/anime atm.
except Kon and Don Kanonji
\"It is much better for you to be feared than to be loved.\"