SwiftKey is one of those apps that pretty much everyone has tried, or at least heard someone mention it. It is one of the most integrated keyboards for your Android device available to date.
With SwiftKey's recent update, they included even more apps to grab ahold of and learn from. This goes a long way ensuring it'll give you the most likely suggestions based on your typing habits and slang.
It's very simple and clean interface can be as modern as you want it, or you can slap a couple themes that are provided for that fruity colorful look.
Option, options, options. That's the key to android, absolute freedom. SwiftKey gives you multiple themes, keyboard size, and keyboard styles. You can even unclip your keyboard and move it to where ever suits your current situation best.
So if you like complete control and high level of predictions for sloppy typers such as myself, I highly recommend SwiftKey