We use these 18V dewalt drills:
http://www.amazon.com/DEWALT-DC970K-...s=dewalt+drill
I don't know how they compare to the others people have mentioned but they work great for us. We have 3 drills so with a total of 6 batteries we can run them nonstop. They have good power(probably not as much as the 20V li-ion) but seems to be sufficient for FRC related tasks.
They will put a few 1.125" holes using a uni-bit into quarter inch aluminum on a full charge but after that you'll need to put in a new battery. we're very happy with them so far.