The more sites the better! I recommend ALL of the major listing sites; Zillow, Trulia, Apartments.com, Rent.com and Craigslist.
There are sites like TurboTenanthttps://www.turbotenant.com/that will allow you to create a property listing and market it to all of those listing sites for you. 30 different listing sites total. One click and done!
I hope this helps save some time and energy for you busy landlords!