Most emitters can be had in bins that fairly close in efficiency/output at the current draw a single cr123 can provide, so it kinda comes down to what sort of beam pattern you're looking for. Domeless LED for maximum throw/hotspot intensity, or domed for a larger hotspot and more intensity in the corona for peripheral illumination.
Best throw would be the osram whiteflat 1mm^2 chip, or its variant (forget the name off the top of my head) that has a generally lower forward voltage and similar output. Could use a cree XM-L for the most lumen output, but that will also give you the biggest/lowest intensity beam for a given reflector/optic diameter.
Single cr123 is probably good for 1.5A at turn on to the chip, 2A at the switch with the overhead from a boost driver. Weaponlight market seems to be moving towards higher and higher max intensity ratings, so i'd weight towards using the osram. Open up the design to allow direct drive off an 18350 and you can get much more imoressive output, but likely unregulated. Which is fine for the use case, most pistol lights will be momentary on, or run for a few minutes at a stretch at most. Rifle/carbine lights see longer periods of use, but still the market seems to want max intensity at turn on.
