If your mind is completely open and you have nothing against Apple and no ties to WP or Android then I'd suggest an iPhone. Not everyone's cup of tea, but the iOS app does seem to get the most love from Spotify.
The Android app comes second - it's getting some attention now and the beta release is more stable, but still missing some important features and releases seem quite slow at the moment.
---------------- Listening on Windows, Android and Sonos. Tweeting it at @davelicence