Pandora. I do have Spotify, and I also have iTunes for my iPod, but I only use Pandora. In fact, I'm listening to it right now. I almost always have it open and playing when I'm foruming, and when I'm playing League of Legends, or Minecraft, or Guns of Icarus, or anything at all on my computer as long as I have my headphones and internet connection.
I would have switched to Spotify, but I find it too annoying to switch over all of my preferences and whatnot, and I only have like 12 songs on iTunes. I also have iHeartRadio on my iPod, but I just use Pandora on there too.