SwiftKey 4 is the latest version of one of the best Android replacement keyboards. The update introduces "Flow", or swipe to type, new gestures and word prediction for 60 languages.
Flow is a that feature lets you slide your finger to the space bar, rather than lifting it and pausing while you're typing a sentence. The refresh also gives easier corrections, and pays attention to your own typing habits, to adjust prediction accordingly.
SwiftKey 4 comes in tablet and smartphone flavours, and it's on sale for $2 (down from the usual $4) for new users. The update is free if you're already using SwiftKey 3. [SwiftKey 4 for tablets, SwiftKey 4 for smartphones]