Ok as one of the few long time airsofters on this board, I think I can answer your question. (I have been playing airsoft since 1998)
First of all is budget.
Ideally I would say "Buy Surefire" but not every one is willing to plop down hundreds of dollars.
However if you hunt, you can find a scoutlight or Millenium weaponlight for a decent price in the market place. There was a M952 for like $150 and a scoutlight for similar money. (albeit it is an incandescent)
Here is a video of me in an indoor warehouse I designed.
http://www.youtube.com/watch?v=oaT4IbPE6go
I used two lights. A surefire M952 weaponlight and a Blackhawk Gladius. (the gladius is the one that does the tactical strobe)
I recommend surefire for a very good reason, if it is good enough for a firearm and the military, it is good enough for airsoft. Also if you watch the video closely there is a moment a guy pops out and shoots at my Surefire. His bb hits my surefire lens and shatters it. No big deal, i call surefire and they sent me a new one free of charge. SF has great customer service. Many other companies do not. so if you want an investment, then invest in Surefire.
For the budget airsofter i recommend the budget firearm weaponlight which is of the following:
surefire G2L (you want the metal bezel)
Malkoff M60
some flashlight mount, there are a ton of inexpensive ones. I would recommend Viking Tactics.
And you are done. This simple setup will cost you about $120 but will crank out a very powerful amount of light. Also you might want to look into rechargable CR123s. These lights eatup batteries.
Depending on your weapon of choice, a remote tape switch may be required.
This also assuming you have weaver rails to mount the light to your weapon.
Hope that helps. If you have any more questions, feel free to PM me.