Short: Lithium Ferro Phosphate - a Lithium variant.

Longer: Of the more commonly available cells, the NimH cells mentioned by Eric are as good as most from a efficiency and sustainability point of view. Low self discharge adds to energy efficiency if you only use them occasionally. If used daily then self discharge is less important.

If you are prepared to use less standard voltages - ie will not drop straight in to a standard torch or radio, then lithium Ion (LiIon) can be good. Energy discharge/charge efficiency is good and if the circuitry is designed well the cycle life can be 1000+ cycles. LiIon are capable of extremely violent destruction if faulty. "Vent with flames" is a cute but descriptive term sometimes used. cf Dream Liner batteries :-(.

Best of all, but less know so far, are lithium Ferro Phosphate (LiFePO4). These are related to Lithium Ion but have an inbuilt crystalline structure which prevents mechanical "pumping" during use cycles and allows very long lifetimes. Cycle lives of 3000 cycles are reasonably easily achieved and larger versions with special care can get 5000++ cycle. Energy out/in efficiency is excellent and they work over extended temperature ranges (say -20C to +60C) and allow higher discharge rates than most LiIon. They are very stable and do not have LiIons melt-down mode. You can drive a spike through their heart and not provoke them to violence. If the dreamliners had used these they would still be flying. LiFePO4 have lower energy densities than LiIon or NimH (larger or heavier for same energy stored) so are less favoured by those who insist on the very lightest or very smallest devices.

Short:

• For equipment which takes "standard size" plug in AA / C / D cells -
  -> Low self discharge NimH cells or batteries.
  Sanyo Eneloop are an excellent example of this technology, but not the only choice.

• For equipment which can be designed to make best use of the battery - and best result overall
  -> Lithium Ferro Phosphate - a Lithium variant.

Longer:

If you want to use cells which can be inserted in equipment designed to use "standard 1.5V" AA or C or D cells, or 9V 'PP3' 'transistor radio type' batteries, then a "LSD" (low self discharge) long cycle life NimH is probably the best choice overall. These provide good sustainability through good use of energy, long number of cycles so less recycling, and relatively benign breakdown products.

There are now a large number of brands of low self discharge NimH batteries available. Of these the best known and best specified and probably "safest bet" are the Sanyo Eneloop cells that Eric mentioned. Others may also be very good, but technical information is usually far less available than for Eneloops cells, and many less known brands will be sourced from equally unknown makers.

LiIon: If you are prepared to use less standard voltages - ie will not drop straight in to a standard torch or radio, then lithium Ion (LiIon) can be good. Energy discharge/charge efficiency is good. Cycle life is usually about 500 cycles, but if the circuitry is designed well and a reduction in available capacity is acceptable then the cycle life can be 1000+ cycles. LiIon are capable of extremely violent destruction if faulty. "Vent with flames" is a cute but descriptive term sometimes used. cf Dream Liner batteries :-(.

Best of all, but less know so far, are Lithium Ferro Phosphate (LiFePO4). These are related to Lithium Ion but have an inbuilt crystalline ("Olivine") structure which prevents mechanical "pumping" during use cycles and allows very long lifetimes, at the cost of reduced capacity per size or weight due to the need to included the electrically inactive Olivine structure). Cycle lives of 3000 cycles are reasonably easily achieved and larger versions with special care can get 5000++ cycle. Energy out/in efficiency is excellent and they work over extended temperature ranges (say -20C to +60C) and allow higher discharge rates than most LiIon. They are very stable and do not have LiIons melt-down mode. You can drive a spike through their heart and not provoke them to violence. If the dreamliners had used these they would still be flying. LiFePO4 have lower energy densities than LiIon or NimH (larger or heavier for same energy stored) so are less favoured by those who insist on the very lightest or very smallest devices.

Here is Eveready's technical information page with links to their various products.
Here is their NimH page - usefully, they list both current and obsolete products, allowing comparisons.

Here is Duracell's product finder with inks to data sheets and more. Also Duracell specification sheets page.

ENELOOP:

The "Candle Power Forums" is an excellent website for a very wide range of discusssions of all things to do with portable lighting including LEDs, batteries and more.
Here is a CPF disucssion on Eneloop - Performqnce details for experts which is useful.

Here is Eneloops home page {USA} with a fairly glossy presenttaion style but some good information.

Note carefully that there are 3 types of Eneloop cells. Standard, XXX and lite. Standard are rated at about 20000 mAh capacity in the AA size (14500) and about 1500 cycle life. The lite has about half the mAh capacity but a 2000 cycle life. The XXX have higher capacity (about 2500 mAh in AA) but about 500 cycle life. The standard and lite versions have extremely low self discharge compared to conventional NimH cells - useful capacity remaining at 3 years under specified storage conditions.

Note that there are 1st and 2nd generatin standard versisons being sold. Buyer beware! - some sellers clain cells are the new version when they are not.
Sanyo say

• 1500 times rechargeable eneloop battery (2nd generation) has a dark gray cover around the positive electrode and a crown logo on the side. HR-3UTGA (AA) / HR-4UTGA (AAA)

• 1000 times rechargeable eneloop battery (1st generation) has a white cover around the positive electrode and there isn't a crown logo on the side. HR-3UTG (AA) / HR-4UTG (AAA)

Short: Lithium Ferro Phosphate - a Lithium variant.

Longer: Of the more commonly available cells, the NimH cells mentioned by Eric are as good as most from a efficiency and sustainability point of view. Low self discharge adds to energy efficiency if you only use them occasionally. If used daily then self discharge is less important.

The new Eneloop batteries have much longer cycle lifetimes that the old ones - the new ones have a "crown" symbol on them. Take great care - many people sell the old style ones and claim them to be new and there are many fakes/clones/ripoffs made.

NimH cells contain Nickel which is not one of the banned "ROHS" materials but has some disposal impact.

If you are prepared to use less standard voltages - ie will not drop straight in to a standard torch or radio, then lithium Ion (LiIon) can be good. Energy discharge/charge efficiency is good and if the circuitry is designed well the cycle life can be 1000+ cycles. LiIon are capable of extremely violent destruction if faulty. "Vent with flames" is a cute but descriptive term sometimes used. cf Dream Liner batteries :-(.

Best of all, but less know so far, are lithium Ferro Phosphate (LiFePO4). These are related to Lithium Ion but have an inbuilt crystalline structure which prevents mechanical "pumping" during use cycles and allows very long lifetimes. Cycle lives of 3000 cycles are reasonably easily achieved and larger versions with special care can get 5000++ cycle. Energy out/in efficiency is excellent and they work over extended temperature ranges (say -20C to +60C) and allow higher discharge rates than most LiIon. They are very stable and do not have LiIons melt-down mode. You can drive a spike through their heart and not provoke them to violence. If the dreamliners had used these they would still be flying. LiFePO4 have lower energy densities than LiIon or NimH (larger or heavier for same energy stored) so are less favoured by those who insist on the very lightest or very smallest devices.

LiIon and LiFePO4 are both environmentally friendly. Discharge them fully (especially LiIon) then drill multiple holes through them and leave soaking in water. Dilute and pour on garden :-). [Lithium can be commercially extracted from sea-water - sending it back there in dilute form is sending it home).

LiFePO4 cost MORE per energy to buy but give the lowest cost of whole lifetime ownership of any available battery, they are about as energy efficient as any, and are probably about as environmentally friendly as you can reasonably get.

Short: Lithium Ferro Phosphate - a Lithium variant.

Longer: Of the more commonly available cells, the NimH cells mentioned by Eric are as good as most from a efficiency and sustainability point of view. Low self discharge adds to energy efficiency if you only use them occasionally. If used daily then self discharge is less important.

The new Eneloop batteries have much longer cycle lifetimes that the old ones - the new ones have a "crown" symbol on them. Take great care - many people sell the old style ones and claim them to be new and there are many fakes/clones/ripoffs made.

NimH cells contain Nickel which is not one of the banned "ROHS" materials but has some disposal impact.

If you are prepared to use less standard voltages - ie will not drop straight in to a standard torch or radio, then lithium Ion (LiIon) can be good. Energy discharge/charge efficiency is good and if the circuitry is designed well the cycle life can be 1000+ cycles. LiIon are capable of extremely violent destruction if faulty. "Vent with flames" is a cute but descriptive term sometimes used. cf Dream Liner batteries :-(.

Best of all, but less know so far, are lithium Ferro Phosphate (LiFePO4). These are related to Lithium Ion but have an inbuilt crystalline structure which prevents mechanical "pumping" during use cycles and allows very long lifetimes. Cycle lives of 3000 cycles are reasonably easily achieved and larger versions with special care can get 5000++ cycle. Energy out/in efficiency is excellent and they work over extended temperature ranges (say -20C to +60C) and allow higher discharge rates than most LiIon. They are very stable and do not have LiIons melt-down mode. You can drive a spike through their heart and not provoke them to violence. If the dreamliners had used these they would still be flying. LiFePO4 have lower energy densities than LiIon or NimH (larger or heavier for same energy stored) so are less favoured by those who insist on the very lightest or very smallest devices.

LiIon and LiFePO4 are both environmentally friendly. Discharge them fully (especially LiIon) then drill multiple holes through them and leave soaking in water. Dilute and pour on garden :-). [Lithium can be commercially extracted from sea-water - sending it back there in dilute form is sending it home).

LiFePO4 cost MORE per energy to buy but give the lowest cost of whole lifetime ownership of any available battery, they are about as energy efficient as any, and are probably about as environmentally friendly as you can reasonably get.

Added:

A good site for battery information in general is Battery University. They provide a wide range of information on more battery types than most people will ever meet.

Most major battery manufacturers provide detailed technical information on web but this can be hard to locate.