Restrictions: Hogyouku Immortality, if they can conventionally kill Aizen, they win, even if in canon he'd regenerate. Butterfly & Monster Forms.
Stipulations: Aizen can make full use of KS.
4th Fusion would have accomplished his goals if Ichigo didn't arrive.Was it Butterfly or Mullet Aizen that was said to actually pose a threat to the RG?
Because if he poses a legitimate threat to the RG he's above all the sternritter combined. Yhwach wouldn't have surpassed him till at least his fight with Ichibei, even then Yhwach didn't have any divine powers till he absorbed mimihagi and the soul king.