Juha Bach.
With all the Sternritter powers, he's near invincible. Hell, with the "Deathdealing" ability, he can't even die.
Let's not forget
"The Visionary" powers that Gremmy had to imagine anything to reality. Like making the opponent's bones into cookies.
"The Wind" ability which will prevent him from getting hit
"The Iron" which makes him invulnerable to physical attacks
"X-Axis" which cannot be blocked
"The Compulsory" which just by looking at something can make it collapse inward.
There's no way Aizen can win.