It's a bad idea to modify any bike like this, let alone a wal-mart bike. Those bikes are designed to be pedal around 5mph in a cul-de-sac and thrown in the trash after a year or two.

You are asking for trouble or catastrophic failure.

The best in what regard? Bafang is probably still one of the best overall. It doesn't have torque assist though. Tongsheng does have torque assist and a lot of people like it.

Then there's more fancy stuff like CYC's Photon which produces 135Nm in 3.5kg.