It won't take my vote for some reason, but if it had...
My vote would go to Spotify as my favorite.
I love that I can listen on demand to something that I don't want to own on CD, but maybe listen to say once every 10 years. Like that time you wanted to hear the theme song to the S Club 7 tv show. And all of their hits. Especially those sung by sweet Hannah, who reminds you a little bit of a younger Baby Spice. Which then makes you wonder if Ginger Spice ever had a solo album. And there it is. It's terrible, but... You nevertheless begin to wonder if Geri Halliwell wants to be your sugar mama. To buy you toys and stuff in exchange for playing with toys and stuff. So... Hopefully, but if not then I (erm, I meant 'you') can still go back to being a thrifty smart shopper by sampling new, unfamiliar music on Spotify in depth before spending your limited funds.
That said...
I also use Pandora (especially if I just want to listen to random genres on shuffle). And I also will stream uploads from my own CD collection on Google Music. Never use iTunes Radio, though I did check it out when it was new.