Since Reinhard couldn't kill Aizen, Aizen would easily win with Kyoka Suigetsu.
In my opinion, it's a stomp, but not really because of that.
Reinhard's arguements are
bullskill yada yada skill yada resurrection yada yada
bullshintuition yada yada, whatever.
Aizen has 4D RW, Reiatsu Crush (which even if Reinhard has resistance, it's a passive 3-C force), and better immortality + reactive evolution that mind you allowed him to go from 4-A to 3-C in three evolutions (if we ignore him going from 5-C to 4-A in one), and Reinhard lacks the means to negate his immortality. KS does have a big role in the fight due to the hyponosis, but Re:Zero characters have that bullshit that their supporters always use where they would just say "Reinhard would just fight with his intuition that allows him to dodge 300 million immesurable speed attacks from all directions with no arms, no legs, all 5 senses cut off, while decapitated and his head on Jupiter", which I would not know the interaction it would have.
Plus Reinhard can't use blitzing amps if speed is equalized because even his combat speed is slower than Aizen's general speed (FTL vs FTL+), and sure his AD/RE would let him grow stronger after he (definitely) dies if he can resurrect, but before that he would either be thrown into Negacion before he can AD fast enough, or Aizen will just use Hado 58 or something to EE him, all of that on top of being caught in KS.
Basically, Rein has no wincons at all because he's being passively withered away and has to deal with hax way above his paygrade while being unable to blitz without evolving, and he will not be able to do that before Aizen puts him in the ground.
Plus I'm pretty sure Aizen isn't getting as utterly skill stomped like Mori Jin vs Saitama or sum shit.