By process of elimination I've narrowed it down to these three: Ulquiorra, Rangiku, and Orihime.
Ulquiorra because he does as he's told when he's told and if nothing else cause even when he doesn't completely finish the job he's efficient. He has no aspirations for a higher position and if he does, he doesn't show it. Basically he just exists. What more can you want from someone that's expendable?
Rangiku because she's so much like me. We can drink, we don't like doing work all the time, we procrastinate and we constantly need someone to keep us on track. But in battle, it's like she undergoes this intense personality transformation and does what's necessary to get the job done.
Orihime because, while I do hate her character most of the time, her heart is ALWAYS in the right place.