What is the most efficient chat api provider that integrates best with Xamarin?

I am wondering which chat API/server provider is most efficient, understandable, cost effective, and easily integrated with Xamarin - the following list:

1.) Buddy - https://buddy.com/
2.) Google Cloud Messaging - https://developers.google.com/cloud-messaging/
3.) QuickBlox - https://quickblox.com/
4.) PubNub - https://www.pubnub.com/
5.) Socket.io - http://socket.io/
5.) Firebase - https://www.firebase.com/
6.) GameSparks - http://www.gamesparks.com/
7.) ShepHertz - http://www.shephertz.com/
9. )Apiomat - https://apiomat.com/en/