I need to generate real Spotify playlists (not a text string that you can drop on Spotify) belonging to a real Spotify-user (me). My tools are .NET 4.0 and Javascript.
It is a web app i'm creating not a Spotify-app living in Spotify.
What is the best/easiest library to use to achieve this?
Where will your app live?
If you're building an app inside Spotify, use this library: http://developer.spotify.com/download/spotify-apps-api/preview/reference/
If you're building a web app outside Spotify, you can try libspotify: https://github.com/jonasl/libspotify-sharp
Be warned, the last link I posted it not up to date.
If you're looking for a web API for building playlists, it's not available now, but it's something we've been thinking about creating.