Hangouts... but it will change because handset makers must now have Duo and Allo included by default.
Hangouts has the sms abilities of messenger just slow and confusing. Messenger has none of the other abilities of Hangouts & Allo. Duo does what hangouts can but only with one contact but hangouts can do multiple. Allo does what hangouts does without the part Duo can also do, while doing what whatsapp can but not everything. So a combination of some or all of these apps are needed to do what whatsapp can in one app for example.
I had to explain that to a colleague the other day. The look on her face was priceless, she asked me to write it down for her and she'll read up on it when she has time.
Now if you need a manual to understand a messenger app you don't have a very user friendly experience do you?
And why on earth is Hangouts Dialer a separate app from Hangouts? This will hurt them in the IM wars