The question really becomes what are you looking for? Some of the services allow you to buy the music (iTunes, Amazon), some of the services allow you to rent the music (Rhapsody, Pandora), and other services are tools to remotely listen to your own music (Google Music, Amazon Cloud Player). Based on past experience, I know that there is a technological revolution just around the corner... bur for now it seems as if we have hit the proverbial wall.
The question then becomes one of hardware. How can you most easily access the music you want to listen to?