Best batteries for a flash?

[fokker]

The amount of current a battery can supply is limited by its internal resistance. Alkaline batteries have a much higher internal resistance than NiMH rechargeables, hence why the rechargeables can recycle the flash faster even though they have a lower voltage rating.

 

[Edsport]

I use 3800 mAh rechargeable batteries. Since they were so cheap i bought a bunch of them on Ebay so i would have lots of backup.
Buy 4 batteries for 99 cents plus shipping. Maybe if you ask the guy if you could save shipping costs if you purchased a few.
New 4x Rechargeable AA Battery 3800 mAh Ni-MH 1.2V | eBay

 

[Big Mike]

I guess we should also mention Lithium batteries. They are more expensive but their biggest benefit is weight...they are very light compared to other types.
This may not be an issue when we are talking about 4 cells in a flash, but if you are packing around 3, 4 or 5 flash units and 10 or more sets of 4, then the weight very well may be an issue. Lithium batteries are not rechargeable though.

There are also the lithium-ion type batteries. These are probably what powers your camera and are obviously rechargeable. They don't make them in standard sizes (AA, AAA, C, D etc) though. This is probably because they use a higher voltage than the other types, and so wouldn't be a good direct replacement for Alkaline or NiMH etc. Also, if they did make them, and you accidentally put one into the wrong type of charger, it might ignite.