Smart Keyboard for 10.9 inch iPad Air 4th generation

Whichever one has the size and weight and cost and features that best meets your particular needs.

“Best” is subjective; a decision and its trade-offs entirely and uniquely yours.

Unique trade-offs? I don’t tend to carry the iPad Pro Magic Keyboard keyboard dock around all that much as it’s substantially heavier, but it’s a great fixed-location dock and iPad stand. You might need the keyboard elsewhere and thus be carrying it more, so...

Here is what Apple offers: iPad Keyboards - Apple

There are third-party products offered too, and most separate Bluetooth keyboards should also work.