As detailed on the App Store description, Keymoji: Emoji Keyboard has had some rather splendid feedback from a number of fairly reputed sources. Even better for you and me, it’s currently down from £3.99 to free. Yoink!
Compatibility: Requires iOS 8.0 or later. Compatible with iPhone, iPad, and iPod touch. This app is optimised for iPhone 5, iPhone 6, and iPhone 6 Plus.