That's a very hard statement to qualify. Certainly, in my opinion, for all the little LED flashlights that use button batteries and are "squeeze to switch on" type, the Photon II is the best.
Of course some people don't like all the fiddling one needs to do to change the batteries. There are four very small screws (which I think could easily be reduced to 2) that you need to unscrew before you can get to the batteries, whereas many of the others merely require a quick pry. But on the other hand, many of the Photon II's competition do not have a "constant on" latch.
The closest and best competition, IMHO, is the CMG 0-4. Same batteries, almost the same size, ability to be constant on, slightly more spunky looking case.
Of course, if you didn't mind paying a little more to get a nice aluminium flashlight that has a much easier battery change procedure, and a more easily and cheaply available battery, with not much of an increase in weight or size, then you should look at the Arc-AAA. But really, I think that falls in a slightly different category.