My mainstays for longevity are:
- Standard Mini Maglite with LED conversion
- Gerber TriTac LED Light
- Gerber Infinity Ultra
- Petzl E+Lite headlight
I have a couple of others that use Luxeon Star LEDs and are ridiculously bright but these typically take at least 6 cells and aren't known for their battery life (though it's still better than incandescent)