Microsoft SwiftKey Keyboard is one of the most powerful and customizable third-party keyboards available. It hit the market several years ago with a predictive engine unlike anything any other keyboard had and the app has grown a lot over the years. It’s a free download with free themes so you don’t have to pay a dime.
Other features include a dedicated number row, SwiftKey Flow which allows for gesture typing, multiple language support, cross-device syncing of your library, and much more. It’s about as good as it gets in the keyboard space. It’s true that Microsoft now owns SwiftKey, but so far they have managed not to mess it up.