I recently bought a FiveMega three-AA body with a SureFire 6P head. It can run any standard module that works with a single Li or Li-Ion battery. Right now I have 3 eneloops in it, which seems to be the best trade-off for brightness (decent current source at 3+ volts), runtime, long shelf life, and rechargeability. At the moment it is driving a Malkoff M30W (Warm-neutral) at about the same brightness I see with an 18650 cell. Of course the ability to run on alkalines is useful in a doomsday scenario too.
I've also experimented with 2xAA eneloop cells in a Malkoff MD3, which also works pretty well at a reduced light output. Surefire 9P can do the same trick.
What do I pick up on the way to work?
I find that I prefer the hand-feel and shorter length of a Surefire 6P, 9p, or Malkoff equivalents. The switches on these lights are superior quality. The 18650, 18500, 17670, and (R)CR123 batteries they support will provide higher current for longer. I have plenty of batteries and lights, there would have to be a month long power outage before I would have to rely on my AA stash. Then I'd be down to 8 lights that use AA, and I can charge AA batteries in my solar powered garden lights. (Hmm, maybe I need to set up a way to charge the Li-Ion cells with solar power too!)
I bought a Romisen NC3-warm light for my mom, it can be used with 2xAA or 1xCR123. She prefers the CR123 configuration. It is more compact, lightweight, and just as bright. I have a bunch of CR123 cells which have a shelf life of 10 years, so she's set for a while.
Beyond pocket lights, the uber-bright 400-700 lumen multi-die LEDs require Li chemistry... I don't want to have gobs of AAs in series due to safety and reliability considerations.
So, I prefer Li battery chemistry, but it's nice to have the option to use batteries from the grocery store.