There are many, many keyboard apps in the Play Store, but Google Keyboard has long been my go-to option. With the Gboard re-branding, Google added a few long-awaited features like multiple active languages and integrated search. That was enough to push it over the 500 million download threshold, joining apps like Photos, Dropbox, and Super-Bright LED Flashlight.
The bracket Gboard has entered is technically 500 million to 1 billion. The page doesn't update right in-time with the numbers, so all we can say is that there are between 500 million and 1 billion downloads for Gboard. This app is helped by the fact that it's pre-installed on a lot of devices, but not all devices like some Google apps. The big OEMs still have their own custom keyboard apps.
Depending on how long you've been following Android, you may remember a time when the Google Keyboard wasn't even in the Play Store. It launched in the Play Store way back in June of 2013 for all devices running Android 4.0 or higher. It compared very favorably to the competition back then, but was a little light on features. After three and a half years and 500 million downloads, it's a fantastic keyboard.