I've used Pandora, Spotify, Rdio, Xbox Music (Now Groove Music), and I find Google Music's recommendation engine by far the best. They even have a "I'm feeling

I've used Pandora, Spotify, Rdio, Xbox Music (Now Groove Music), and I find Google Music's recommendation engine by far the best. They even have a "I'm feeling lucky" feature that plays a set of curated music to your interest randomly.