Looked this up and sorted based on robbery rate: https://en.wikipedia.org/wiki/List_of_United_States_cities_by_crime_rate
It looks like the population number cutoff to be included as a city is right around 215K people.
Among that size and larger, the lowest rate of robbery of any city in the US is--drum roll, please:
Irvine, CA
Also lowest in the nation for violent crime in general.
And this isn't some "ivory tower"... Irvine is in Orange County which has a collective population of over 3M people, and Orange County has the 4th-lowest violent crime rate per 100K population in the entire state of 58 counties.