I have a UTG tactical that is attached to one of my home defense guns and I swear by it. Here is a newer version.
http://www.leapers.com/prod_detail.php?mitem=flash&level1=LED_Flashlight&level2=&itemno=LT-ELP116Q&status=&mtrack=1m
In light of the other conversation, I would look for a few things.
1. High lumens (200 plus recommended but even my 90 is extremely bright)
2. Strobe feature, very disorienting to an attacker.
3. Heavy steel construction and defense protrusions around the light (see the image in the link above). These are great if it gets down to you smashing the attacker’s face with the flashlight to cause even more damage. A small feature like that is one you hope you never need but glad you had it in the very off chance you need it.
4. Option to mount on your gun if you see fit. Look for one that fits or comes with weaver mounts or whatever system your primary home defense weapon has.
5. Comfort in carrying. Does it fit easily in your pocket? Will it fall out?
I’m sure there are a lot more features others will bring up.
Strobe and brightness both indicate and electronic light. There is no reason to buy a nonelectronic light anymore (one where the on/off switch simply connects the battery to bulb with maybe a current limiting resistor aka passive electronics).
Active electronics allow the light to amplify the meager current from a "dead" battery. I've got a supply of dead AA batteries (from other gadgets) that I can't possibly use up. My light (Maratac) just keeps going and going with those dead batteries.