Friend ask me which Rechargeable for his Flash. I am not too sure which is the best at this moment since technology keep improving, but I would like to recommend the one I am using (Sanyo Eneloop).
What makes Eneloop different from others?
Sanyo Eneloop is low self-discharge NiMH battery (LSD NiMH). It reduces self-discharge and, therefore, lengthens shelf life compared to normal rechargeables. It's voltage is more stable, reduced heat buildup when the battery is quickly charged or discharged and higher efficiency. Which make it good for photography and other high energy requirement applications.
I have been using Sanyo Eneloop for my TV remotes, my camera flash and toys over a year. All I can say is, this batteries has been doing so good and none of them fail on me.
If you wanna give it a try, Amazon has the whole product lines available here.
Also, I find the La Crosse BC-700 Charger maybe the perfect charger for the Enoloop. Amazon has it for $26.82 with Free Shipping.