You question is far too general. If I asked the same question about the DFW metroplex, what would your answer be?
If you said Garland or White Settlement, or University Park, then correspondingly here in Atlanta I would say Stone Mountain, or Douglasville or Home Park.
And neither one of our answers would have any meaning because quantity is relative to demography, property type, price range and a host of other relevant, hyper-local, socionomic factors that only matter to the individual investor.
So, what type of rental properties are you talking about?