Ask youself what you plan on using it for and where. Then try to standardize on one type of battery to make resupply easier. My standards are AA and CR123A's. Since I travel the subways alot, I need a strong light. Also, a strong light is a force option (blinding). Here is what I have:

On me: Streamlight TL-3, Streamlight 3AA 1 Watt Luxeon Tasklight. (Carry extra CR123 batteries as these things go out on you without warning when they run out)

PSK: Princeton Tec Yukon HL (1 watt Luxeon head lamp with 3 secondary LED lights)

B.O.B (sits at home and never sees daylight unless I check on the status of the equipment) 3 LED rechargeable light with built in hand powered crank.