I use Eneloop batteries. They're great as they have very little idle discharge, so you can leave them on the shelf for months and they're still good to go when you need them.
One more for Eneloop.
Just avoid "lite" versions, these are not for intensive use.
Emil Pozar, http://emil-pozar.photoshelter.com/
After researching the various options I've stuck with Eneloops. There are higher-capacity batteries but there are downsides to using them. Eneloops are the best combination of price/capacity/reliability/shot-to-shot speed.
6,560 FLASH POPS LATER: The Results of "AA" Rechargeable Battery Tests http://photography-on-the.net/forum/showthread.php?t=677074
Why Eneloop and Powerex? http://eu.nkon.nl/waarom-eneloop-powerex