This may seem an odd one as my choice is a cheap Chinese mod, the Noisy Cricket 2-25.
Its the fact it can run in 4.2V, 8.4V bypass or in Variable Voltage with the potentiometer dial with basic safeties. I have mods that can do either but the NC2 is the only one I have that can do all three and, as I'm not much of a squonker or TC vaper is the only mod I own that caters for every type of vaping I regularly use.