Well some years back it was MS Combat Flight Simulator 3. Out of the box, it was fairly bad but I fell in with a bunch that taught me how to properly revise the flight models for the stock and many, many great add on aircraft for the sim and it was completely addicting. I ended up making and testing the flight models for almost every WWII fighter every used in the war and many of the bombers and utility aircraft. Actually got to some amazing degree of accuracy in the flight characteristics, traded noted with actual warbird fliers etc. Then I got too busy with work, feel out of touch, suffered some computer replacements and never got back to it.
I would like to see a good Napoleonic sim.