Depends on why I can only have one, and keep in mind I'm still 2 weeks from being able to order anything (yey new job!) so I may change my mind later, but I can guess from the reviews and such I've seen so far.
Now:
Infinity Ultra if I need small. From what I hear either the Arc AAA or Infinity will render my solitair as usefull as a match stick, but the Infin has a longer life. And slightly bigger wouldn't bother me that much anyway.
Realisticly, my minimag with a BB400. Feel funny about not being able to adjust the beam, but from the looks of the beamshots I won't need to. Or possibly a LSH-P. Anyone know of a side by side between BB400 or 500 and the LSH-P?
If the poop has hit the global fan, and survival is more important then wether lunch will be at McDonalds or Burger King, a 6D maglite w/ EverLED. Long, heavy, metal tube that happens to double as a flashlight.
Later:
LS4 or LS5. No depends. If I need small, I'll suck it and go with CR123. Realisticly, 2xAA pack (lit is just too expensive for me, and I don't like short/fat). poop, fan, I'll carry a big stick and whack them after I blast them with the burst mode.
Overall (obviously) LS4 or 5 w/ 2xAA. Diffrent settings for diffrent uses, goes down a notch automaticly when the batteries are just too drained, 'find me', and I'm long used to carrying my minimag on my belt every waking moment. And from what I hear, even the AAA is built to last longer then I will. This thing is going to be sexyer then my last girlfriend (I won't say current /ubbthreads/images/graemlins/grin.gif).