you have to just say charcter vs character. Not just say their spirits so it doesn't count. if you do you destroy the entire fun of this thread.
As far as power goes Kisame and Gai ( with gates open ) would match the shinigami well.
I would also like to see a full power Itachi vs. Aizen. Since I look at Aizens release as a type of genjutsu ( in the narutoverse of course)