Rechargeables are nice. That said, the best battery I've come across in the AA form factor, performance-wise, seems to be the Energizer Ultimate Lithium. Previously known as the Energizer e^2 lithium, and before that version, they were the redtop Energizer AA's.
I've used them in all of my flashes since 1993, and in every camera's motor drive as well. Very fast recycling in flash units, and a slight increase in fps in the motor drives I've used them in. On my FM2n's MD-11, I went from 3.2-3.5 fps to about 3.8 fps. The MD-4 that I use with my F3P is around 4-4.3 fps now, instead of 3.8 fps.
-J