If you fancy using Google's Messenger app, you'll be happy to know that it recently got an update that adds the ability to use quick-reply from notifications.
Though it's undocumented in Messenger's changelog on the store, you can see the feature in action above. When a new message comes in, the notification tab now has a reply button. When a user taps the button, they can then reply to the incoming message without ever opening the full app.
It's a simple addition, but a pretty nice one nonetheless, and gives the Material Design-heavy Messenger app a slight leg up over Hangouts, which is still lacking a similar feature. The update appears to be making its way out to users now, so if it doesn't show as available for you just yet, sit tight a little longer.