Nicad They are the most expensive as well.

To clarify, NiCD batteries do have the longest overall lifespan in terms of charge cycles, as compared to Nickel-Metal Hydride (NiMH) or Lithium-Ion (Li-ion). However, NiMH batteries have a higher capacity per charge.

NiMH lasts longer than the others like NiCad and Li-Ion. NiCad develops memory when you charge it teh wrong way that is charging it even if it doesn't need to be charged.

There are also Lead-acid batteris that are rechargable, i.e. the one in you car. These do larst a long time, but for small applications are impracticable. On lithium-iron, they only have a (typical) life of just over 8years. Whether you use them or not.

You didn't mention a specific application, so it is difficult to provide a recommendation. The battery that lasts the longest is the one that is matched best to the application. Different battery technologies respond in different ways to the way in which they are used. The highest energy density (watt-hours per unit weight) of common commercial battery types is provided by Lithium-Ion (Li-ion), followed in order of decreasing capacity by Nickel-Metal-Hydride (NiMH), resusable alkaline, Nickel-Cadmium (NiCd), and lead acid. In terms of cycle life (number of times the battery can be recharged), the ranking from best to worst is: NiCd, Li-ion, NiMH, lead acid, and reusable alkaline. NiCd and, to a lesser extent, NiMH batteries have a problem with battery 'memory'. If you repeatedly recharge the batteries from a partially-discharged state, they will lose their ability to hold a full charge and will not be able to discharge as deeply. This is why NiCd batteries can appear to fail so quickly - don't top them up, let them discharge completely. NiCd batteries have the shortest charging time and lead acid the longest. NiCd also has the widest operating range with respect to temperature. Lead acid batteries require routine maintenance and last the longest if they are 'topped up' on a regular basis. If they are stored in a partially depleted condition, they will sulphate - reducing their capacity and life. Because of the memory issue and high levels of self-discharge, NiCd batteries require the most maintenance to get the most out of them. The cheapest batteries are NiCd and the most expensive Li-ion (cycle cost, not retail price). NiMH and reusable alkaline do not handle high load curents as well as the others. Lead acid, on the other hand, handles high current draw very well, which is one of the reasons it is well suited for use in automobiles. All heavy metals have environmental considerations. Batteries must be disposed of properly, since materials such as lead and cadmium are toxic. Don't just pitch them in the bin, take them to a recycler. In general terms, Li-ion batteries have a lot going for them. They are pricey, but perform very well in many consumer applications. NiCd batteries remain in widespread use because of their low operating cost.

One disillusioning thing I learned about rechargeable D batteries is that the cheaper ones are just "C" batteries with filler. On the other hand, maybe that's all you need. If you have an application that uses a LOT of current, pick your brand and then get the most expensive variety within that brand. Radio Shack, for instance, has several varieties. I don't think rechargeables will hold a charge as long as alkalines, though, so they are best for things that you not only use a lot of current in, but use often. I play a lot of cassette tapes and my player has a low-battery indicator. I also have a battery tester. This way I am always able to fully discharge one battery at a time--test it to see which one has gone dead--and recently threw away the last of a set of four that I bought thirteen years ago and have used regularly ever since. (They happened to be GE.) They really do recharge a thousand times and in fact are far more likely to get lost than they are to wear out.

There are two different answers because battery usage falls into two different categories. One is portable lighting and the other is everything else. Everything else is covered by answers given ( lithium the best, and Nicd last ). These also lose a charge in storage where after a few weeks, you need to recharge. While these batteries are high capacity, they run out of juice without warning and that is fine for everything else. Lighting is flashlights witch alkaline is the best (correct voltage and long storage). I mention this because of the popular explosion of led lighting. Most of them employ chips that have different settings that are voltage dependent. Most take three batteries at 4.5v vs 3.8 which renders your high setting useless. Most portable lighting uses can be bad if they run out without warning. Alkaline run down at a steady pace, getting dimmer and dimmer which is a realtime indicator of your battery state. Rechargeable alkalines are the best for portable lighting that is used regularly every weekend and they store as long as regular alkalines (five years). Who wants to grab for a portable light after a month and find it discharged or have it run out abruptly from bright to dark?

The answer depends a lot on how you define "lasts longer". Each of the batteries have different characteristics when it comes to how much power they are capable of holding, how fast they loose their charge if they are fully charged and then not used, how long they will last if you discharge them slowly, (like in a clock), how long they will last if you discharge them quickly, (heavy load like a digital camera). They are also effected very differently by temperatures, both when they are charged and when they are used. The life and capacity of all of them can also be drastically diminished if they are charged improperly, or discharged in series with batteries that have unequal capacities. The simplified but incomplete answer among commonly available batteries is that NiMh batteries are the best choice for most people who want a high capacity battery for high drain equipment like digital cameras. They will provide more life than NiCad batteries if you use them very soon after you recharge them, at normal room temperatures. However they loose their charge even if they are not used at a very fast rate, (15% in 1 day, 30% in a month). NiCad batteries hold their charge longer, (even though they start with less), and work better at lower temperatures, like when it is freezing outside. If you want a clock to last forever you are probably still much better off with a regular non-rechargeable lithium battery, because none of the rechargeable batteries last as long without occasional topping off. If you use any appliance once or twice a year, rechargeable batteries are also not a good choice, since they are expensive and will probably not last 2 years even though you rarely use them.

I was amazed with NiMH, other rechargeable batteries have been a complete joke compared to them.