You know, I've strangely preferred the Htc stock keyboard, however I am using Swiftkey. Its prediction is fantastic, and it's design is outstanding. The prediction on any keyboard, gets better the more you use it. You might notice in all of these replies, most everyone says how they always go back to this keyboard, or that keyboard. Well... It's true. Just use SwiftKey, you'll be glad you did.
Oh, and I just thought I'd throw this out there, I absolutely hate the Google keyboard. I've heard awesome things everywhere, but when I tried it, I was surprised. It was nothing more than a plain white keyboard, that looked as good as my Android phone in 2009. Boring, dull, and highly limited functionality. Yay, a keyboard that looks outdated, and still remembers how to correct my bad spelling. Neat!
-Gold made Sense, on HTC's One M8