SwiftKey 4 (Android)

Pros: Excellent word prediction, gets better at predicting the more you use it. Can predict words of up to three languages simultaneously. New flow style of typing works really well with word prediction.

Cons: Getting used to using flow after being used to traditional typing takes a little time to get used to.

Summary: Probably the best android keyboard available. The best feature about it is it's ability to learn from the words and sentences you use to type and adapts it's prediction technology to choose which words you're likely to write next. The more you use it, the better it gets at predicting words. New version includes flow for people who like to swipe words instead of typing.