I thought I'd delve a little deeper into the world of custom and high-end flashlights.
The first question on everyone's mind is money. How much do you want to spend? There's great lights available for $50 or $100. That might seem like a lot to spend on a flashlight, but just like a pocket knife (we'll get into those later), you don't realize how often you'll use it until you have one. You want sometime that's water resistant at the minimum (if it keeps water out, it keeps sweat, dirt, grit, pocket lint, all of that out), and runs on batteries that you can find pretty easily.
There's two routes you want to consider - do I want to buy something from an online retailer, drop the batteries in the light from the box, and have a great flashlight? Or do I want to buy a light or light body (a "host"), pick a drop-in LED module, pick some rechargeables (if you'd like), and build an incredible light?
That's not to say that there isn't off-the-shelf lights that are shit-hot awesome in terms of output and runtime, but sometimes building it yourself from a few select parts turns out a really nice light.
If you're looking at off the shelf, here's some brands to consider - Fenix, Quark, 4Sevens, NiteCore, Novatac, Malkoff, SureFire, and HDS Systems. Those are pretty much ascending in price, but give google a shot and have a look around.
If you're a custom kind of guy, and are considering throwing something together that meets your specifications, you're going to want to consider SureFire's line of products, but buying a light just to pull the LED module out and drop something else in. Two kinds of SureFire's you need to worry about - 6P/C2's, and E2E/E2D's. One's got a slightly larger head, the other slightly smaller.
The 6P uses 2 batteries, it's bigger brother the 9P uses 3, and it's slightly different cousin, the C2 and C3 lights, all use the same sized dropin. What's known as a P60. Malkoff, Nailbender, and a few others are relatively inexpensive (think $60), and have an absolutely incredible output and runtime.
Either route you take, you should probably ask yourself whether you want to use AA, AAA batteries, similarly sized rechargeables, or CR123's, the industry standard. All are available at Wal-Mart, but CR123's are going to run you a little more. Better to buy online.
But this is just a rather introductory post - you should really head over to candlepowerforums.com if you'd like to do a bit of research yourself.
I'll post some pictures in the next few days to show you what you're looking at.
If you have any questions, comment!