I have chosen the 1xAA sized lights as my standard. If you buy the right lights, you can use Alkaline primaries, rechargeable NiMH's, Lithium primaries, and rechargeable Li-Ion's in them. Lots of options there. If you're in a pinch you can run to the grocery store and buy some alkalines cheaply. While many grocery stores also have CR123's these days, you will pay through the nose buying them there. Plus, you can go to Costco and buy one of their mega-packs of AA Alkalines for real cheap and have a lot of backup batteries on hand.

I have a couple of 1xAA lights (4Sevens, Olight) that are every bit as bright, if not brighter, then my 2xCR123A light (Chinese light from Deal Extreme - model WF502B). This is with two primary CR123A cells in the WF502B and a Li-Ion cell in either the 4Sevens or the Olight. And the WF502B is huge compared to the other two lights.