I've not seen a generator (quite likely 'cause I've never gone looking), but I've seen model plane motors claiming almost 95% efficiency (a quick hunt found this one claiming 94% max efficiency: http://www.astroflight.com/index.php?main_page=product_info&... )

I'm guessing that's pretty reversible - if you spun that motor backwards with the right speed and torque and loaded the output up in just the right fashion, I think you'd get 94% of the energy you put in out as electricity.

The problem I see is that a slowly falling weight is unlikely to provide "the right speed and torque" without some sort of lossy gearbox in between - there's the physical equivalent of an impedance mismatch there. If I had to imagine a motor/generator that'd be likely to work on a direct-drive to a falling weight it'd probably be several feet in diameter. (For model plane motors, the diameter of the motor is a significant factor in the kV constant and the rpm at max efficiency. Small diameter motor spin fast, large diameter motors spin slow.)