It sure depends on a loooot of variables... how big, how heavy, how fast and how much you are looking to spend.
For example, Ebike sized stuff? Brushless is a near no-brainer, but for car and motorcycle sized stuff, brushed can be a good setup for cheap horsepower. it all depends on what you are planning to do. As far as efficiency goes, it also depends on the motor design, brushed motors like the Lemco and Agni can be more efficient than most poorly designed chinese brushless motors, on the flip size, the most efficient motor in the world IIRC is brushless (Csiro).
The same applies for batteries, how much for how long and at what cost?
for ebike sized stuff, ping makes a good low-C rate pack that are pretty reliable and have a great cycle life. Headways are slightly better discharge wise, but can get bulky in a hurry, they also have a decent cycle life. Cell-man at EM3EV.com makes an excellent A123 26650 pack that is awesome for high-cycle life, high-discharge and ok energy density, but big packs can be a little costly (Worth it, as is the rest of the stuff he sells). Turnigy packs are an awesome bang-for-your-buck battery, lots of discharge capability, super modular and very lightweight. Cycle life can be in the mid hundreds if treated right but they will explode and burn your house/workplace/apt down if you mistreat them.
