I like Textra -- but my daily driver is still Evolve SMS, which has had Material Design for a bit now -- if you're a paid user and have access to custom themes, you can access the material design UI via the hidden settings. This works really well with the numerous excellent Material Design themes (from Ryan Borden and others) available from Google Play.
Another app that has material design that I use a lot is Flip, which is a currency converter app -- it's probably the nicest-looking looking one on the market: https://play.google.com/store/apps/d...ton.flip&hl=en
Android Devices: Moto Droid, Samsung Galaxy S, HTC Desire Z, Samsung Galaxy SII, Sony XPeria Arc, LG Optimus 2X, HTC Desire S, HTC Sensation, Google Nexus 7, HTC One M7, Sony XPeria Z1, Samsung Galaxy S5; CURRENT DEVICES: Nexus 5 d821, HTC One M8
iOS: iPhone 4s, 5, 5s, iPad Mini with Retina Display