The Energiser Lithium Ultimates are excellent but if you need a AA sized battery which can stand extreme temperature variations i.e. down to -50 C and which can left and forgotten about but need to work in the most extreme environments then these could be a solution
http://www.buyabattery.co.uk/saft-ls14500-battery-3-6v-lithium-lsg14500-size-aa-cr14500.htmlLithium-thionyl chloride cells have a high energy density, partly because of their high nominal voltage of 3.6 V. Bobbin versions can reach 1220 Wh/L and 760 Wh/kg, for a capacity of 18.5 Ah at 3.6 V in D format. Because self-discharge is extremely low (less than 1% per year), this kind of cell can support long storage periods and achieve a service life of 10 to 20 years.
http://www.saftbatteries.com/Technologies_Lithium_LiSOCl2_303/Default.aspxTypically the Saft cells have twice the energy storage of the Energiser Lithium primaries and excellent low self discharge giving useful storage lifetimes >20 years.
Pricing is typically equivalent to the Energiser Lithium Ultimates, when considering the energy storage per cost.
Energiser Lithium AA @1.5V 3200mA will give 4.8 WHrs
Saft LS14500 AA @3.6v 2600mA will give 9.4 WHrs
or Twice the run time for similar battery weight.
Being a 3.6V terminal voltage you need to make sure your flashlight i.e. a single AA flashlight can take some of the rechargeable 14500 Lithium Ion cells. Also make sure that the flashlight LED emitter doesn't consume more than 0.5W i.e. a typical Cree 50 Lumen flashlight.
These batteries are typically the Saft ones found in emergency location beacon transmitters (ELTs, EPIRBs)
The main downsides, the Saft batteries could explode if short circuited. So they need a little more respect.