Verizon Messages. It's the closest thing to iMessage for me in terms of syncing and doing texts across my Galaxy S6, iPad, and Mac. I can send SMS texts directly over Wi-Fi when i'm not on my phone using my Verizon number. Plus the app is now Material and extremely customizable.
Funny that Hangouts could never do this.
I also use WhatsApp when i'm interacting with friends and fam overseas.