I am not at all sure that Aizen Created all the Arrancar.
(1) Aizen asked Number One the Lonely wolf Guy to Join him, he was already his lonely split in two self.
(2) The Old Guy was already his Skeletal age everything self.
3) Don't really know about the Shark Girl
I'd like to believe that Ulqiorra was already his powerful self, complete with V2 before Aizen recruited him. My opinion is that Ulq's 2nd release may indeed have been more powerful than the top 3 Espada, but we'll never know for sure.
I suspect that Ichigo's Super Hollow V2 would be needed to stop Aizen or Aizen+Hougyoukyuu, if this is so, it might indicate that. Ulq V2 wasn't too shabby.
EDit: Yes Ulq V2 is definitely more powerful than the Captains except for Captain Commanda Genrusai and perhaps Kyouraku Shunsui.