I have been on Android since the days of the Samsung Epic, Moto droid, & my go to text/IM app has been Handcent now known as Next sms. Its awesome in the way you can customize notification icon & sound for each contact as well as customize the the text bubble for each contact and backgrounds..
That being said i have been using the google messenger since i got my Nexus 6P and i am really liking the clean simple interface,
I am not a fan of Hangouts but do like the Google stand alone Messenger.
I barely use Handcent anymore..