If you like enough power and reliability, light weight, get a Tanaka 40. With a mountain bike your option will be a rack mount.
Friciton drive-easy to install simplicity, robs power, not good in rain
Golden Eagle= belt drive,dependable quite drive train, relativel easy to install, only single speed, relatively more expensive.
Staton Chain Drive-Have new 7 speed drive train, good on hills, fast on flats, dependable, lifetime warrany on gearbox. Downside hevier than most,one of the more expensive, in my opinion you get wht you pay for.