I am pretty happy with the Tongsheng tsdz2. It is a relatively easy one to install. The motor isn't too noticeable. It is also easy to use, just pedal.
As far as the top speed, it can be set in the menus. However, it isn't a high-speed unit, it just makes riding easier.
I have had this unit, a bafang mid-drive, and a geared hub motor. Comparatively, this one is the easiest and most natural to use.