The RF500TB is a very efficient, powerful, slow pancake motor.
Mabuchi makes some of the best volume-manufactured motors in the world. They're usually a premium over "clone" motors, but for good reason.
This is an example of their excellence - the RF500TB-14415. We brought these in for a local teacher's project to be mated with propellers for a wind-generator project (yup, DC motors can be used as generators too).
It's nominally rated 5V, but has a full operation range of 1.5 to 9V