I have tried many different keyboards over the past couple years. The one that I have always stuck with was swiftkey. That has always had the best word prediction in my opinion. However, recently I signed up for the beta version of Fleksy keyboard. It is being developed for use by blind people. This keyboard has unbelievable word prediction. It is so good that you can literally type without even looking at your phone. It is worth trying at the very least. Give it a shot!
Edit: Dev link http://fleksy.com/