Been doing these for a few years now. Up until now I've always let the kids choose their own projects. This time I "encouraged" one I wanted. Which AA Battery lasts the longest. Why AA? We use them the most - remotes, Wii controllers, etc. Here are the Challengers:

Duracell Coppertop
Energizer Max
Rayovac
Heavy Duty by Rayovac (we use these in cheap toys, etc)
Energizer Rechargable 2450mAh

Put two of each into a small flashlight. Turned them all on at the same time with a clock next to them to monitor the time.

The winner? Energizer Max.