courtesy Groovy Green
Popphoto has published a good comparison of rechargable AA batteries, testing them on digital cameras to see which were able to hold the biggest charge and which performed the best. They were also compared by charger type and against traditional "disposable" batteries* .
It’s uncertain whether these values will translate into optimal performance for your gadget or flashlight, but click the photo or link above to check out their results.
*Please don’t throw away batteries. Check with your trash/recycling hauler or local stores, many of which now will take your dead batteries in free of charge (no pun intended!).