I still really miss Google Hangouts. It was integrated with Gmail as a web interface. The web and mobile app synced perfectly. You could use SMS/MMS, or chat, seamlessly in the same interface. Contacts worked correctly. SMS and chat history saved to Gmail so all the text was searchable. It was AMAZING and Google killed it for no reason, only to later replace it with inferior options.

Edit: oh yeah, you could also make phone calls, or group voice and video calls using the same app/web interface.