I prefer AA for flashlights, because it is a size common for most of my electronic stuff, you can get alkalines cheap, NiMH rechargables are readily available, you can splurge on lithium AAs for an application that needs it. AAAs cost as much as AAs, but have much less capacity, so I don't see a use for them, except for small form factor items [EDC pocket light]. CR123s are relatively expensive, I have nothing else that uses 123s, so I have no reason to buy a flashlight that uses "special" batteries.
EssLight