Stark's probably number one. He'd be a great opponent for some combination of Urahara, Ukitake, Kyoraku, or Isshin Kurosaki (while Ichigo takes care of Aizen or something). My guess is that Stark's el primero, Halibel's la segunda, and the unnamed one's el trecimo. And I'm also thinking that we could have a Yoruichi vs. Halibel and a Yamamoto vs. Evil Abuelo in our far futures, as it's been mentioned elsewhere.
I'm personally more concerned about the fights that are going on right now. Are they really going to kill off three Espada at once? Doubtful. My guess is that Noitora and Zomari cut out early, but Szael Apollo's going to go down to some ****ing bizarre supernatural pseudo-scientific nonsense.