This important thread was resurrected and cleaned up by was.lost.but.now.found but has been a little neglected and was starting to show it's age so here is yet another update as we try to keep this relevant, accurate, and useful.
As always, comments and suggestions for improvement are welcome.
Ze.
To all new members: Feel free to copy and paste the questions below into a
NEW THREAD (no need to reply on this one) when you are seeking advice on buying a light. Don't worry if you don't know the answer to every question, this is simply a way for us to see what criteria that are important to you. Also, the more answers you provide, the better the responses from experienced members will be.
==================Flashlight Recommendation Checklist================
1) How would you prefer to purchase the light?
____I would like to purchase the light in person from a brick and mortar store. I am located in ______________.
____This will be mail-order or Online (location doesn't matter).
2) Budget: An easy question, but you may change your mind after answering the rest! 
____I don't know yet
____Up to $25.
____Up to $50.
____Up to $100.
____Up to $200.
____Up to $300.
____Essentially unlimited.
3) Format:
____I am not sure, please help me decide.
____I want a flashlight (hand held/self contained).
____I want a self-contained headlamp.
____I want a headlamp with an external pack/power source.
____I want a mounted light (typically for a bicycle or vehicle)
____I want a lantern/area light.
____I want a portable spotlight (it may have an external power source).
____Other ____________________________________________
4) Size:
____MICRO - Keychain size.
____TINY - Every day carry (2-4 inches).
____SMALL - Every day carry (4-7 inches).
____MEDIUM - Holster/belt ring carry. (>7 inches)
____LARGE - Big enough to need its own travel case.
____I don't know/I don't care.
5) Emitter/Light source:
____LED (known for efficiency, longevity, and compactness)
____Incandescent (known for superior color rendition)
____HID (known for max output, but often at the expense of size)
____I don't know.
6) Manufacturer:
____I want to buy a light from a large/traditional manufacturer that is ready to go out of the box.
____I would like a light from a specialty manufacturer (Possibly limited run/Custom).
____I am interested in assembling my own components. (for example a "host" or flashlight body from one manufacturer, and a "drop-in" emitter from another source).
7) What power source do you want to use?
____I intend to use "Primary"/Disposable
Alkaline batteries based on the usual AAA/AA/C/D sized cells common to most stores.
____I intend to use "Primary"/Disposable
Lithium batteries based on the usual AAA/AA/C/D and CR123 sized cells common to most stores (often a cold weather or long storage choice).
____I intend to use
Rechargeable cells (NiMH or NiCD) based on the usual AAA/AA/C/D sized cells common to most stores.
____I intend to use
Rechargeable cells based on less common formats (18500 or 18650 Li-Ion, RCR123, et-al).
____I want a light with an integrated rechargeable battery pack.
____I don't know/I need more information on power sources.
7a) If you have selected a rechargeable option
____I want a light that plugs directly into the wall (literally with prongs built into the light)
____I want a light that has a recharging adapter (your typical "wall wart")
____I want a light that snaps into a cradle (usually mounted on a wall)
____I want a separate/stand-alone charger (this involves removing the batteries to charge)
____I don't care
8) How much genuine out the front (OTF) light do you want/need? Sometimes you can have too much light (trying to read up close up with a 100 lumen light is not a happy experience).
____I want to navigate a dark room or read a map (1-10 lumens).
____I want an indoor "blackout" light (15-50 lumens)
____I want to confidently walk around an unlit/unpaved rural area (60-150 lumens).
____I want to illuminate my entire backyard or a campsite (150-300 lumens).
____I want to illuminate an entire field, the neighbor's front yard several houses down, impress my friends and neighbors, etc. (300-700 lumens).
____I want search and rescue type illumination (800+ lumens).
____
SPECIAL NOTE: Burst/Turbo mode Category - There are several lights that will run at a super bright maximum for a very limited period (usually 5-10 minutes) and then will "step-down" to a lower level for thermal control. Check here if this is acceptable.
9)
Flood vs Throw: Flood covers an area, Throw reaches out to a distance.
____All Flood: I am doing "arms length" tasks like reading and campsite cooking.
____Wide Flood: I want a defined flood area for semi-close tasks like after-dark campsite tasks or working on a car.
____Narrow Flood: I want a sharply defined flood area that will project some distance for tasks like trail walking.
____Wide Throw: I want a beam with a noticeable hot-center for distance throw and a significant amount of "side-spill". Good for rough trail hiking, search and rescue, and general distance work.
____Narrow Throw: I want a beam with a very tight "hot center" and minimal "side-spill". Good for distance viewing, fog, and looking through dense undergrowth.
____Turbohead: I want a far-distance projector with a sharply focused spot of light and minimal or zero side-spill. Good for extreme distance and impressing your friends.
9a) Distance: How far away will you typically need to see with this light (check all that apply)
____Less than 1 yard/meter (reading, other close work)
____Less than 5 yards/meters (looking for something inside a dark shed/garage/basement)
____5-20 yards/meters (check out a noise in the backyard)
____30-50 yards/meters (I have a big backyard)
____50-150 yards/meters (I live in a very rural area/farm with wide open spaces)
____150+ yards (I am searching from a helicopter)
10) Runtime: Not over-inflated manufacturer runtime claims, but usable brightness measured from first activation to 50% with new batteries (Measured on maximum continuous output).
____Up to 30 minutes (I want the brightest [and potentially smallest] light for brief periods)
____30-60 minutes (I have plenty of batteries just ready to be changed)
____90-120 minutes (Runtime is moderately important, but still not critical)
____3 hours + (I critically need this light to run on max for extended periods in between battery changes/charges).
11) Durability/Usage: Generally the old phrase "you get what you pay for" is very accurate for flashlights.
____Not Important (A "night-stand" light).
____Slightly Important (Walks around the neighborhood).
____Very Important (Camping, Backpacking, Car Glove-box).
____Critical (Police, Fire, Search & Rescue, Caving, Survival).
____I don't know.
12) Switch Size, Type, and location (choose all that apply):
____Any size switch will do.
____I need a BIG switch (I'll be using gloves or have very large hands or coordination issues).
____I want a forward clicky (Helpful for momentary activation and signaling).
____I want a reverse clicky (For use with multi-mode/level lights).
____I want a momentary switch (Predominantly for use with signaling and short bursts of momentary light only).
____I want a twisty switch (Tighten the head/tailcap to activate, and the light will stay on until the head/tailcap is loosened).
____I want a body mounted switch (near the head, like on a Maglite).
____I want a tail mounted switch (found on the majority of today's high end lights).
____I want a remote switch (usually found on high-end bicycle headlights)
____I don't care.
____I don't know.
____Other, please specify____________________.
13) User Interface (UI) and mode selection. Select all that apply.
____A simple on-off with only one output level is fine for me.
____I want 2 light levels. (Brighter/short runtime and Dimmer/long runtime.)
____I want multiple light levels. (Some lights have 5-16 light levels.)
____I want a programmable light.
____I want a selector ring.
____I want a strobe mode. (Oscillating pattern to confuse/blind aka "Police Mode")
____I want SOS mode. (blinks in ---...--- emergency pattern)
____I want a beacon mode. (Regular flashes at full power to show location.)
____I don't care.
____I don't know.
14)Material/Finish/Coating
____Plastic/composite body (this may limit your choices significantly).
____Anodized Aluminum – either type II or III (Hard Anodized) (Aluminum, specifically HA, is the most common material/finish for today's higher end flashlights).
____Stainless steel (durable, but much heavier than aluminum)
____Titanium (durable and nearly as lightweight as aluminum, but can be moderately to significantly more expensive).
____I don't care.
____I don't know.
____Other, please specify____________.
15) Water resistance
____None needed
____IPX4 (Splash resistant)
____IPX7 (Waterproof to 1 meter/30min)
____IPX8 (Submersible to greater than 1 meter for 4 hours)
16) Storage conditions
____In house (temperature/climate controlled environment)
____Emergency kit (long standby periods)
____Automobile glove-box (wide temperature swings, long standby periods, critical reliability)
____Other_____________________________________________
17) Special Needs/extras: Is there anything else you want or need that hasn't been mentioned? Select any/all below.
____Red filter (for preserving night vision).
____Other filter colors (Amber, Green, Blue, _________).
____Dedicated R/G/B secondary LEDs.
____"Hybrid" light (bright incandescent combined with long running LEDs)
____Pocket/belt clip
____Holster
____Wrist/Neck Lanyard
____Crenulated bezel
____Non-sparking, Intrinsically Safe (IS) for use in explosive environments
