best practice is

• the flashlight should run on AAs because they're the most common
• you should have some non-rechargeable lithium AAs for it (not rechargeable lithium-ion!) because they're much better to store long-term than alkaline batteries
• if you actually use it regularly, also have some rechargeable nimh batteries (eneloops or whatnot) to use in it

Of course, in the grand tradition of advice being "do what I say, not what I do," I should admit that I do not own any flashlights that run on AAs. It is good advice though.

When it comes to anything important that you aren't using constantly, if it has to be charged to work, you should assume that when you need it you won't have remembered to charge it. And for anything that runs on rechargeable batteries, remember that those batteries will sooner or later die. And if you forget to recharge them over a long period, it will be sooner, because being left in a fully discharged state will kill them for sure.

This applies doubly to anything with a built-in, non-user-serviceable battery (which is why I hate that shit), but it applies...singly, I guess, to things with rechargeable non-standard batteries like enthusiast flashlights that run on lithium-ion cells.