Amazon, Windows Azure, DigitalOcean.
Amazon is pretty expenesive, pretty fast, Windows Azure is… Windows which might not be for everyone taste (though it’s really easy to manage and save money if you are not using it to much), DigitalOcean is pretty cheap and good, but lack the real enterprise support of former two.
From all of them I would choose Windows Azure, if not for other reason then because it’s already battle tested in game development (Titanfall, Halo 4).