This is a tough question, and there is no easy answer: it depends partly on the circumstances and needs of the client, whether the investor wishes to manage himself/herself, where you believe the best prospects for appreciation are, opportunity, future family needs?...
RealtyTrac puts out a good study on their web site (see "RealtyTrac study on rental condition") showing that the best performing markets for rental returns are:
Baltimore, Maryland: 24.82%, followed by:
Clayton, GA with 24.26%,
Wayne, MI: 21.08%,
Pasco, FL: 19.2% .....
The markets with the lowest return on investment are:
New York County, NY, with 2.34%, followed by:
San Francisco County, CA: 3.2%,
then Kings County/Brooklyn, NY: 3.63%,
Marin County, and close behind San Mateo County and Santa Clara County, at 4.31%
... and Santa Cruz County at 4.54%
Return on investment is defined in this study as gross return: rental price divided into the purchase price. There is a lot more that goes into a more real "net" return: you have to take into account expenses, which include such costs as: - management company, - repairs, - vacancy, - taxes & insurance, - wear and tear on the house etc ... More on this subject with this "net operating income" link, noted above.
Thank you for reading,