I can make a strong recommendation for the Sanyo Eneloop batteries - these are 'hybrids' with a much longer shelf life than normal NiMH batteries, and hold 85% of their charge for 12 months. They are more expensive than standard NiMH, but worth the extra especially if you want to keep a charged set as backup.
I actually have 48 of them!
The prices vary a lot - this is the best place I found...
Instant