optics, which provide a smooth beam pattern ideal for fine investigations
high CRI (color rendering index)
As of my current knowledge, the previously mentioned Nitecore MT06MD, and the Lumintop IYP365 is also high CRI (edit: go for the Nichia 219B version, which is high CRI and has a “warmer” color temperature (4000K) - do not even consider the XP-2G version).
Here is a group buy thread (you can save a fairly good amount) for the P20: