The D-9 is a very challenging aircraft to use. It can roll very well, climb very well, accelerate very well, zoom climb very well, maintains E very well, the guns are good (bad ballistics???), and the views are well enough (though not as giving as others). I think the only plane that turns worst (sustained) is the 190A/F-8. Take away its speed and it is much easier to deal with.
Take speed away from the La7 and P51x, and they both can still dance.
The Spitfire Mk XVI is by far the most agile plane in the game, and it is fast too. Its worst attribute is its longevity in the fight thanks to limited ammo. That mostly comes in to play vs bombers though.