If you have one light, you want it to be a headlight. It is much easier to hold a headlight in the hand than to fix a flashlight to the head, and there are situations where you do not want a light on the head. The ability to switch modes rapidly is very useful. My basic light is a headlight.

There are AA headlights using single batteries. Consider Zebralight - http://www.zebralight.com/. They aren't cheap, but I have found their headlight to be satisfactory, once I modified and simplified their headlight strap. Check out candlepower forums for similar brands - there are quite a few.

One feature of the Zlight that I really appreciate is that it comes with a clip that allows you to attach the light to clothing so that you have both hands free and depth perception capability.

I have more or less standardized on AA, with some AAAs in the mix. Primarily, I like my GPS and light(s) to share the same batteries. Right now I can go afield with either a AA set, or a AAA set, or conceivably both. What is really critical if the ability to recharge, which I can do in either house current, 12V DC, or solar. The solar option adds weight and bulk, critical in backpacking, but not to a totally unreasonable degree.

I note that I am beginning to acquire more and more units, charged through UPS technology, that use 18650 batteries, which well might replace AAs as a standard.

Battery standardization is a worthy goal, but probably unattainable in the real world. The alternative is organization, not something that comes naturally to me.