The ZebraLight SC51 or SC51W would be my first choice. Maximum utility and choice in brightness vs. runtime with standard AA batteries & the best UI. But if its switch or electronics fail for any reason you're SOL…
So if I had to have a single light that I was sure to be reliable under any situation, I'd probably pick my Malkoff M30W w/ VME head in a Surefire LX2 (bored) host. I'd trust that to always work, the LX2 tailcap gives a two-stage setting, and with a couple of appropriate battery spacers on-hand, it'll run on 17670, CR123, RCR123, 14500, or any other AA (or even AAA) battery. Truly the most versatile & bullet-proof package IMO.