If there's one locally, check prices at a model airplane hobby shop. They should have a wide selection of rechargeable batteries, both drop-in AA replacement and with solder tabs, and will know a good bit about them since they're now used as primary flight power for many radio control airplanes.
If you go with NiMH, you'll need a charger that's designed for that type cell, since overcharging them will destroy them in one go (NiCd are somewhat tolerant of mild overcharge); the charger must sense voltage, look for the "peak" and completely stop, rather than drop to trickle as most older NiCd chargers will.
The other battery type you might consider if you can get one that will fit your battery compartment is lithium ion/lithium polymer -- these have almost the energy density of alkaline primary cells (2-3 times NiMH, about 5 times NiCd), but are rechargeable and much lighter than NiCd or NiMH type as well. They cost a lot, but they last as well or better than NiCd and will have a lower lifetime cost because they don't require recharging as frequently (battery life is measured in recharge cycles) and don't self-discharge (which greatly reduces recharges required for batteries used sporadically).
Ansmann is only a "private label" - they buy in bulk from a number of makers and stick their name on the cells.
You never now beforehand what you buy in that pack... it may be decent, it may be crap.
The motor for my RTS takes twelve double A's. They last along time but I'd like to find a more environmentally friendly way to do things. When you dispose of that many batteries in one shot, It makes an impression. Maybe I should try the new rechargables or a _really_ long extension cord. :)
Battery 101.... NiCd or Nickle Cadmium batteries are the cheapest form of rechargeble battery. But, they are a problem with equipment designed to use regular batteries as they are 1.2 V per cell as opposed to 1.5 for standard carbon cell battery. As well, they exhibit "memory". Simplified that means if they are not constantly totally discharged eventually they remember the lesser charges we give them. However, on the other hand, if you run them down flat they can be ruined too.
NiMH or nickle metal hydride batteries are/were designed to overcome the above problems. Higher voltage and supposed lack of memory effect. But,they need a different charger. Duracell makes a nice combination one.
If your unit is a high discharge device like a flash or motor drive, go to the NiMH. They are a better investment in the long run.
Now, lithium batteries (Lithium Ion to be correct) are the "cat's ass". But the cost will never justify them in normal use. They have a high discharge, good recovery rate and long time between recharge. And, they need a special charger.
Now, a really dirt cheap alternative. There are "enercell" green batteries, which are normally just alkalines that can be recharged. They are cheap, come with a charger pack to start and are direct replacements for alkalines. However, I find you will get about a year of use and recharge before they show lack of respect... I used them, because they were cheap. Dated them, and tossed them when they were a year old. But, eventually converted everything to NiMH. Personally I don't recommend them
It is all up to the amount of current that the device takes. Low current devices like light meters, radios, etc. can take just about anything. Extreme low current devices like watches used to take mercury, but environmental concerns changed that to silver halides. High current devices like flashes need the good stuff. Motor drives actually fall in the middle. And, for you digital folk, digital cameras are high current and need the good ones!
I'm with Panchromatic, on this one. I use lithiums in everything. They last much longer, the flash recycles almost immediately, and they are impervious to cold.