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. 

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Added:

A good site for battery information in general is [**Battery University**](http://batteryuniversity.com/). 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.