It really depends on what you are looking for in a third party keyboard? Do you need a word swipe feature? Then Gboard might be a good option. For a long list of keyboards check
this topic out and click on the ones which you like to see the App Store description and more on features and to see when the latest updated date was.
I'm still switching between
Swype,
Gboard and
Path Input (
iPhone version,
iPad version). Neither off them is perfect though.
Some people tend to like
Nintype as well but I find it a bit weird and isn't a thing for me although it does offer a word swipe feature which I'm especially liking and the main reason for using a third party keyboard. There is also
Microsoft Wordflow (only available in US).
Go Keyboard is also pretty nice and regularly updated.