Generally the best NiMH batteries are the low-self-discharge varieties. These basically keep their charge intact even when they aren't used. But if you are using a lot of batteries now, it's unlikely that they will lose their charge since you will be using and replacing them too quickly. I'd just get the largest capacity NiMH batteries you can find, and if you think they'll bit sitting around a lot, then get the low-self-discharge version.
For chargers:
The cheapest NiMH chargers are the very slow trickle chargers. These just charge the batteries endlessly forever and are the slowest, cheapest and hardest-on-the-battery variety of charger. These will charge a battery in 8-12 hours.
After that comes the timed NiMH chargers which basically charge continuously until they reach a timed limit and then shut off. These chargers also are cheap and hard on longterm battery life since, unless the battery is nearly dead, you'll overcharge it, but they are much faster than the trickle chargers. They charge a cell in about 2 hours usually - but results vary and the cells usually aren't fully charged.
Above that are the so-called "smart" chargers which use a delta-V charging method that watches for a voltage drop that indicates the end of charging. These chargers are usually not "cheap" but are not too expensive, charge quickly and are pretty good for keeping batteries alive for a while. These charge batteries at varying speeds - the fastest ones are the hardest on the battery life. They can be as short as 90 minutes... less than that becomes very hard on the batteries due to outgassing and eletrolyte breakdown.
Next are the cycling smart chargers - these discharge the battery completely before starting the charging cycle. There is no "memory effect" with NiMH cells, but cycling them does seem to help make them last longer and store a larger charge. These cycle (drain and then refill) a battery in about 3 hours.
Then there's the delta-T chargers - these use a rapid temperature rise to determine the state of charge. They usually need special batteries, charge extremely quickly, and usually cost a lot. They are not too hard on battery life, but can be fooled by temperature variations and have been replaced by delta-P chargers mentioned below. They can completely charge a depleted cell in less than 30 minutes.
The most expensive kind of NiMH charger nowadays is the delta-pressure variety. They watch for the beginning of the hydrogen-generation stage of charge in order to terminate the charge. All NiMH batteries vent gasses when overcharged, these chargers sense this and cut-off charging. These charge extremely quickly (like 15 minutes or less), require special cells, and are usually pretty expensive. Since they are very precise, they are the best type of chargers for long battery life and they are very good at charging cells completely since they shut off right at the peak charge without overcharging at all. The best known brand of delta-P charger is the Rayovac I-C3 charger/battery (
http://www.rayovac.com/technical/wp_ic3.htm ). As mentioned, with both delta-T and delta-P chargers, the cells need to be special cells that are matched to the chargers. You can't buy a delta-P charger and use regular NiMH cells - the charger will ignore other types of NiMH cells.
The great NiMH battery shoot-out:
http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM
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