nIKON SB910, wHICH BATTERY ?
eneloop XX cells are probably the best overall combination of battery life and shelf life. They're 2500mAh.
You may get slightly more life out of Sanyo 2700mAh cells if you use them immediately after charging. But if you want to be able to just charge up some batteries, leave them in the flash, and have the flash work nearly to capacity a month or two later, then XX cells will retain far more charge for a longer period of time.