I prefer flashlights that use form factors that are easy-to-find worldwide.
AAA (and one coin cell) light on my body.
AA (and one 9V, plus a few extra AA and AAA) light in my everyday bag.
I voted for AAA because no one else had, although it’s mentioned in several comments here.
Voted for AAA size, but AA size is also OK.
I don't really need a flashlight with super duper long throws and/or that last many many hours or days on a single cell/battery. But some of the AAA and AA based lights available now can last quite a few hours and have reasonable throw (70 ~ 100 m).
I prefer flashlights to be small, and that use commonly available cells/batteries such as the AAA and the AA. So if I need extra cells/batteries in a hurry/emergency, I just go to the nearest local store and pickup some more AA's or AAA's. Other cell/battery sizes such as CR123, 18650 and 26650 are either too expensive and/or not readily available from local stores.