Minuum is a unique keyboard with a single row layout and a wonderfully robust prediction technology. Really conserves the onscreen space. Could be uncomfortable with fat fingers. Still in beta. Lags badly on low-end devices.
When it comes to predictions, Swiftkey rules supreme. Recommended for tapping typists. Learns from social accounts and saves this knowledge in cloud for quick relearning. It lags on low-end devices, though.
The best of all worlds. This keyboard has qwerty, T9 and compact qwerty layouts at disposal. There are themes, gestures, predictions and dictionaries. Very responsive and fast. Good for tapping typists.