I live in Washington State and invest here. I consider it one of the top four most landlord friendly states (with the exception of the city of Seattle which has a strong tenant union and some laws that put landlords at a disadvantage).
Here are my top four: Louisiana, Texas, Utah and Washington. Not necessarily in that order.
Other states that have been known as landlord friendly: Indiana, Wisconsin, Colorado, Arizona, Florida, Kentucky, Georgia, Mississippi, Virginia, Arkansas, Nevada.
From what people have posted on BP, I'm guessing some of the least friendly are California, Massachusetts, New Jersey, New York and large metropolitan areas such as the Chicago area of Illinois.
One of our most useful tools in Washington is the 20-day Notice to Terminate (no-cause) that can be used with month-to-month rental agreements. Also evictions can take as little as 30 days from start to finish. (Note: Seattle has "Just Cause Eviction Protection" and is not landlord friendly.) Also, we do not have rent control.