I use GPM, but that's because of the bundling with Youtube Red. I would prefer Spotify for a desktop app, nicer looking UI IMO, and so Google doesn't know my music prefs. But for now, ad-free YT is too nice to give up.
What operating system? I found Google Play Music Desktop Player to be pretty awesome (although it's just a browser underneath but it has much better integration with the rest of the system through themes, media keys) also native last.fm, lyrics, etc.
