Based on ROI, I have experienced Renton and SE Seattle to be excellent. Prices are still lower than nearby areas, and the fastest appreciateing zip codes in 2012 were in these areas. Rent rates are great due to easy access to Seattle jobs, and with Renton, you have decent access to Seattle and also to the Eastside.
Prices in Seattle and on the Eastside (Bellevue, Issaquah, Sammamish, Kirkland, and Woodinville) are higher, so getting the cash flow for good ROI is more difficult.
I work with investors and have two years of training in which I immmersed myself in the study of building a real estate investment business.
Call me with your further questions.