The "dryest" battery sucker upper I have heard of is from the modder "milkyspit" and his "milky candle", which leaves very few electrons in the 123. I think he is considering another run of these. - not really a flashlight.
IMHO, the best way find a light which really drains the 123, is to buy one with at least 3 cells, and ideally 4 in series. The reasons are:
- It is very hard for electronics to work with voltages < 1 volt, and especially under 0.7 V
- It is easier to obtain maximum overall efficiency from a buck converter than a boost converter. Even these, often require the V bat > Vf LED + 1 volt.
- At 1 volt, a 123 is effectively completely dead (some will argue this point, but you are really close to drained). 4 cells x 1 volt = just enough to drive an LED with a little margin.
I think that there are only a few lights in this category due to the length. Surefire makes some, and I think the Aleph (see the sandwich shoppe) also comes in a 4 x length.