Well, this is why I like Zexion:
He's smart and takes a non-confrontational approach to problems, but can fight if he needs to. Also, out of all OXIII members, he would be the most interesting to fight. He has controls the 'Illusion' element and fights with a gigantic book.
Think about that for a minute. Controlling Illusions, he can clones himself if he wants to swarm you, he could send you into a massive loop of hallucinations if he wants to torment you or incapacitate you, and he could copy any weapon he wants which means he could assumingly attack with Axel's Chakrams, Larxene's knives, Marluxia's scythe, Xigbar's Gun Arrows, or any other member's wepaon for that matter.
Then, there's that lexicon. He can blast you with it, have it blast you, or have it create a vortex to suck you in. Then, once he traps you in it, he could confuse, baffle , and bombard you in his very own dimension to his heart's content. Also, if the book works the way I think it does, then Zexion is probably one of only two members (the other being Marluxia) that can instatly kill you if he traps you in it, and then destroys the book you're in.
Zexion is cool, and don't let his intellectual approach fool you. He could own you if he wanted; he just chooses not to. It's all part of his illusion.