I’d hate to say it but the Apple keyboard. I tried the Belkin keyboard for 9.7 pro and it was bulky, occasionally lost its Bluetooth connection and you never knew when the battery would fail.
When I got the 10.5 Pro I went for their own keyboard which connects to the terminals on the side which it communicates through and draws it power. The keyboard is also very nice to use and plenty of shortcuts to speed up its use. When not in use it folds up like a thicker cover, noticeable but not uncomfortable.