The MM Plus WO would be the brightest IIRC, and you'll get more current to your LED from 1.2v nimh with the lower VF led you can use.
So say you got a L vf, the emitter will only get say 100ma, and then you got a H vf emitter, it might get 350ma or more... Get it?
So use the lowest Vf emitter you can and you'll get the most current to it possible.
Or you can try contacting MillerMods for a board.. He can make a high output one, and probably has stock fenix boards from pulls that he might sell you that would have a better comprimise of runtime...