KMC e-bike chain 10sp or any KMC to suit your cassette because they have been proven in testing to have the least stretch and wear compared to the competition,i have always used them on S pedelecs and have had no problems.
I just buy any 9 speed chain for mine and they all seem to last ok. Mechanic at LBS says the motor smooths out the torque so chain wear isn't the problem you might think it is...