When I design Android apps, I use HVGA resolution in the emulator. I saw (on YouTube and other videocasts) that some developer use higher resolution like WVGA or similar.
Which resolution is best for designing nowadays Android apps? Why?
Why restrict yourself to one resolution? You are perfectly able to customize the design for all screen resolutions. See this page in the developer docs: Supporting Multiple Screens
It tells you how to support multiple screen sizes and how to test them.