The brightest headlamp that uses AA batteries is the Fenix 15-UE.
This LED headlamp is capable of outputting 900 lumens in “burst” mode — but only for 30 seconds at at time (it must cool down afterwards).
However, in “high mode” it outputs 400 lumens and lasts an average of 3.5 hours in this mode.
The headlamp has regulated output, so it doesn’t dim as battery voltage drops, however the higher brightness settings eventually become unavailable. In mixed usage consisting of approximately 75% medium and 25% high, the high setting becomes unavailable after about 5 hours using 2300 mAh batteries.
The Fenix 15-UE weighs 5.4 ounces (154 grams), this is excluding batteries and headlamp straps.
The photo at the top of the page shows the 15-UE in “burst mode”. Here is an example of it in action in “high mode”:
The maximum beam reach of the 15-UE is 583 feet (178 meters). The headlamp is IPX-6 Water Resistant –basically, this means it is splash proof but not submersible.
The LEDs used in this lamp are Cree XM-L2 U2 — these are the highest-performing, commercially available single-die LEDs.
The Fenix 15-UE is available at Amazon for around $63.