While I personally also find Swiftkey to be the best for pay keyboard on Android, I recently found a very good free one:
"Multiling Keyboard (new beta)" which is totally amazing, 320KB in size plus language packs so it comes in around 1MB vs. Swiftkey 16MB, no network permissions (language packs are on the Play Store), and probably the most customizable keyboard with a big number of tricks up it's sleeves (custom auto-completion of text blocks, swipe, custom keys, adjustable keyboard position, etc.)
I think this one is best for people who are concerned about privacy, since it has no network permissons.