I'm sure there are better lights out there for much more money, but I use DX part number 39736.
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The only downside is that this emitter is limited to a 4.2V, or I would swap out the body of this WF-502B light for a WF-502D light.
I've got a Klarus XT11 on order, so I'll soon know if the $63 lights are better than the $21 lights.
Regardless, I'll probably stay with P60 drop-ins. I've found that weapon-mounted lights last as long as the weapons (i.e., many, many years...I still have a few old shotguns with 12 LED lights on them.) That means that LED and battery technology will advance A LONG WAY before you want to replace the physical mounts or the tube that holds the light together. A replaceable drop-in makes upgrading easy.