The brightest headlamp that uses a single AA battery is the Zebralight H52 (Cool White version). It shines with 300 lumens on its highest mode.
This headlamp weighs just 1.1 oz (32 grams), or 2.9 oz (80 grams) with an AA battery and headband. This makes it a great choice for a runner or night hiker.
In most cases, you’ll want to use the 116-lumen “medium” mode — this is quite sufficient to light up a trail at night. The Zebralight H52 lasts about three hours in this mode, when using a rechargeable NiMH batteries like the Eneloop Pro. The headlamp is also capable of accepting a 3.7V 14500 lithium-ion rechargeable battery.
When in high mode (300 lumens mode) it lasts about 40 minutes. In low mode it outputs 27 lumens — this enough light to read a book in bed for example.
The maximum beam length is 66 meters.
The Zebralight H52 is waterproof to IPX7 — this means it can be submersed for 30 minutes at a depth of 1 meter. This is due to its unibody aluminum casing.
Here are some more nice features on this headlamp:
A battery capacity indicator (LED flashes 1-4 times)
Automatic stepping down to lower output when battery (AA and 14500) capacity is low
Over-discharging protection for 14500 batteries at 2.8V cutoff