The best choice is to have the mass of the motor attached to the bike’s frame for higher wheel maneuverability, especially for mountain biking and full-suspension bikes.

These motors are typically more complex and heavier due to the required mechanisms that separate the pedal crank from the  chain ring. This is necessary to let pedals and motor drive the rear wheel through the bike’s chain independently.

Bosch, Yamaha, Shimano, Brose are well-known brands offering this type of motor, however they only produce motors for OEM e-bike fabrication. They all need a special frame design to fit this motor.

Bafang or 8FUN is a well-known factory which manufactures mid-drive motors for regular bikes retrofits. They require standard bottom brackets to be fit on. Transmitting the muscle power and motor power through the existing chain and cogs is problematic for bicycle conversion which reduces the reliability and durability, hence it is not recommended for urban or commuter bicycle conversion.