I have from my own experience found that
1) Form factor,
2) battery type,
3) run time and
4) light quality
are the most important criteria to me.
As a side note I no longer purchase 1x123 lights, it must be 2x123 as I use 17670; for one battery lights I use 1xAA. I don't use rechargeable 1x123 as is against point 3
Light quality comes with experience of different lights, I pretty much narrowed down which brands I purchase, regardless what other xyz brand is coming out with or what features they have. Form factor is something personal that also comes with experience, Underwater Kinetics LED 4C side by side is a great form factor for me. Run time - there is a company that knows how to do that best. Last my favorite lights are 18650