I've been having a blast recently with the Plane Design Spitfire. Its an FS9 aircraft but easily ports over to FSX. Overall my favorite Flight Sim add on is the RealAir Spitfire, but the Plane Design Spit is a close second due to having the most detailed model/textures of any add on I have seen, and also due to its excellent Merlin engine sound set and challenging flight model. It actually has the most challenging flight model I have encountered, which makes it extremely fun to fly, if you can get the hang of it.
The first pic here is a stock aircraft with a 1710hp Merlin engine.
This one is my modified version, where I upped the boost from 18lb to 25lb, which has over 2000hp. Its also stripped of all military extras to make it as light and quick as possible. One thing I like about Flight Sim is all the control you have over engine and aircraft variables.