FS 2004. I just installed it yesterday. Overall I'm impressed, and I've played every version since the IBM 8088 days. Learned to fly on it, actually.
They have "improved" the weather by making it more realistic, or so they say. You need 512MB of RAM to make it function, however. Leaving the weather less realistic runs fine on my 1.4 gig Athlon with 256MB of Ram and a GeForce 4 card.
A big problem is that it still doesn't do gravel runways very well. They are more like trying to land on a river bottom. The plane bounces all over the place and has way too much resistance from the gravel. The DC-3 is fun to fly, I haven't tried the Wright Flyer yet.
The 777 still sucks. You can't view your HSI and power guages at the same time, unless you use the virtual cockpit view. My 17" monitor doesn't do that too well.
The helicopter is much easier to hover, and the caravan seems to fly better, the old version it would glide for miles with the power at idle.