For DIY the most popular systems with a torque sensor are Tongsheng (too many problems for me) and CYC, while Bafang makes the BBS02 and BBSHD which are RPM based. FME, AFICT (and I could be mistaken), the torque based ones provide assist based on the pressure you exert on the pedals, while RPM based on the speed you're pedaling. The great feature of the DIY systems is the amount of assist for each level can be programmed specifically for each individual.