What is Asimov's best book?

They discover that the Machines have generalized the First Law to mean "No robot may harm humanity, or through inaction, allow humanity to come to harm". (This is similar to the Zeroth Law which Asimov developed in later novels.) Dr Calvin concludes that the "glitches" are deliberate acts by the Machines, allowing a small amount of harm to come to selected individuals in order to prevent a large amount of harm coming to humanity as a whole.