Spotify, even though it KILLS me that they replace parentheses with brackets. I can't listen to any songs with features because it will mess up my last.fm
I don't have Spotify and I've never used Pandora so I can't fairly vote, but I must admit that the first time I used iTunes Radio, all of my faves were played in a row and it was like a magical moment.
But I've only used it that one time since I like controlling what I listen to.
But Spotify Radio requires a plan to use it on mobile devices. iTunes Radio is portable on phone and computer.
But having the Premium service enables one to listen to basically any track/album they want no matter what device they're on. Also, I really can't just look at one aspect of Spotify while disregarding another, so with you saying you're only specifically talking about the radio factor, I have to include the other facets of Spotify Premium as well.