From Businessweek:

Dallas-Fort Worth

Rank: 1
Price/rent ratio: 3.76*
Median per unit sales price: $30,000
Median rent: $720/month
Vacancy: 7.6%

Pros: Investors in the Dallas metro market can earn the highest rental income for the least amount of investment. 
Cons: Dallas-Fort Worth has a tendency to have large waves of apartment construction, which can rapidly increase competition and push up the vacancy rate.

*The ratio represents how many dollars of investment will generate $1 of rental income. The lower the ratio, the cheaper the apartments are in terms the rental revenue. The national price/rent average is 8.19.

Cleveland

Rank: 2
Price/rent ratio: 3.94
Median sales price: $31,000
Median rent: $701
Vacancy: 6.4%

Pros: Solid rental income for cost of investment.
Cons: Rising unemployment and generally weakening economy.

Detroit

Rank: 3
Price/rent ratio: 4.00
Median sales price: $34,000
Median rent: $765
Vacancy: 7.4%

Pros: People are staying in apartments because the foreclosure-heavy for-sale home market is increasingly risky.
Cons: The economy, heavily dependent on the automobile industry, is crumbling as unemployment spikes.

Cincinnati

Rank: 4
Price/rent ratio: 4.15
Median sales price: $34,000
Median rent: $671
Vacancy: 7.3%

Pros: Steady market with a limited supply of new apartment construction.
Cons: Job losses, particularly in manufacturing and trade/transportation/utilities sectors will curtail apartment demand over the next year.

Houston

Rank: 5
Price/rent ratio: 4.52
Median sales price: $35,000Median rent: $718
Vacancy: 10.1%

Pros: Strong employment growth over past decade; many major corporations headquartered here.
Cons: Dependent on oil market, which has weakened. Relatively high vacancy rate.

Charlotte

Rank: 6
Price/rent ratio: 4.85
Median sales price: $38,000
Median rent: $714
Vacancy: 8.5%

Pros: Several years of steady economic strength, rent growth, and relatively low vacancies—plus it's a major college town. The market could do well in the long term.
Cons: The local economy of this banking hub has suffered greatly since the financial crisis began. Layoffs and unemployment are on the rise.

Jacksonville, Fla.

Rank: 7
Price/rent ratio: 5.17
Median sales price: $41,000
Median rent: $757
Vacancy: 12.7%

Pros: The city continues to attract retirees, and the job market benefits from the development of the nearby naval station. The area is also home to several universities, including Jacksonville University.
Cons: The financial sector has been hit hard in recent months and values of apartment buildings have been declining as vacancies climb.

Kansas City, Mo.

Rank: 8
Price/rent ratio: 5.19
Median sales price: $37,500
Median rent: $656
Vacancy: 8.2%

Pros: New apartment construction has been limited, which has kept supply in check.
Cons: Job losses have been rising along with apartment vacancies.

Columbus, Ohio

Rank: 9
Price/rent ratio: 5.43
Median sales price: $38,000
Median rent:$636
Vacancy: 8.3%

Pros: Columbus is home to Ohio State University, the nation's largest campus. The college creates a steady flow of tenants.
Cons: The local economy is weakening and risks for landlords are increasing outside the student zone.

St. Louis

Rank: 10
Price/rent ratio: 5.43
Median sales price: $41,000
Median rent: $685
Vacancy: 8.2%

Pros: New construction of apartments is limited.
Cons: Unemployment is increasing. More than a thousand layoffs were announced as a result of the recent takeover of Anheuser-Busch by Belgium-based InBev.

Minneapolis

Rank: 11
Price/rent ratio: 5.43
Median sales price: $56,000
Median rent: $901
Vacancy: 4.7%

Pros: The city is home to the University of Minnesota—a steady source for tenants. The metro has good long-term prospects because of its diverse employment base.
Cons: Home of Target and many financial companies, the city is seeing its economy soften.

Philadelphia

Rank: 12
Price/rent ratio: 5.49
Median sales price: $61,000
Median rent: $986
Vacancy: 6.1%

Pros: The city has a relatively stable economy with many universities and hospitals. It is also the sixth-largest city in the U.S. with the fourth-highest gross domestic product.
Cons: There have been some job losses, particularly in manufacturing and professional/business services sectors. Vacancies have increased and this trend is expected to continue throughout the year.

Milwaukee

Rank: 13
Price/rent ratio: 5.68
Median sales price: $52,000
Median rent: $797
Vacancy: 4.3%

Pros: There is a limited amount of new construction and vacancies are low.
Cons: Significant job losses this year could reduce demand for apartments.

Boston

Rank: 14
Price/rent ratio: 6.12
Median sales price: $113,000
Median rent: $1,644
Vacancy: 6.4%

Pros: A major real estate market with many universities and strong rental income.
Cons: The city has had job losses, especially in the financial services sector.

Indianapolis

Rank: 15
Price/rent ratio: 6.15
Median sales price: $43,000
Median rent: $635
Vacancy: 8.2%

Pros: Stable apartment prices and relatively strong job market.
Cons: New apartment construction has accelerated.

Orlando

Rank: 16
Price/rent ratio: 6.40
Median sales price: $57,000
Median rent: $824
Vacancy: 9.9%

Pros: Growing population and good long-term demographic trends.
Cons: The hospitality industry has been hit hard and Disney World has been laying off. Owners of unsold condos have been competing with traditional apartments for tenants.

Chicago

Rank: 17
Price/rent ratio: 6.52
Median sales price: $73,000
Median rent: $992
Vacancy: 6.0%

Pros: The Chicago area has seen steady long-term rent growth and provides good rental income for a large market.
Cons: The Chicago financial industry has suffered job losses. New construction of apartments has continued.

Tampa-St. Petersburg

Rank: 18
Price/rent ratio: 6.56
Median sales price: $57,000
Median rent: $798
Vacancy: 9.3%

Pros: New construction of apartments has slowed.
Cons: Weakened job market. Condo owners who are unable to sell homes are competing with apartment owners for tenants.

Washington

Rank: 19
Price/rent ratio: 6.57
Median sales price: $97,000
Median rent: $1,306
Vacancy: 5.9%

Pros: D.C. is one of the nation's few economic bright spots because of its federal government and defense contractor jobs. Apartment building values are holding steady.
Cons: The northern Virginia suburbs, which have an oversupply of apartments, are getting hit harder than the urban core as vacancies rise and unemployment grows.

Fort Lauderdale

Rank: 20
Price/rent ratio: 6.74
Median sales price: $78,000
Median rent: $1,038
Vacancy: 7.1%

Pros: Vacancies are relatively low; the location will continue to attract retirees and others who enjoy the warm climate and the beach.
Cons: Job losses, unsold homes competing for tenants with apartment buildings, condos being converted into rentals.