Well, you'll have to be a bit more specific. A replacement to keep using Hangouts without having to, well, use Hangouts? Or a IM messaging replacement? Or an overall SMS messaging replacement?
It seems many don't agree with you since Hangouts has been generally well received, but in my case, I seldom use Hangouts AT ALL. None of my friends use it, so... For an IM-type messenger, I'd recommend Whatsapp and Facebook Messenger (yeah, I know, the fuss of the buyout, blah blah blah). Some friends use Line, but I personally don't like it. Skype also has built-in messaging, but I just don't like the invasive interface.
Oh, and I'm not sure there's a third-party app that allows you to connect to Hangouts... maybe apps like IM+ but I'm not sure.