Flight-Gear is, well i would say the best public flight sim with X-Plane coming second. Reason i say this is that Flight Gear is used by some large aviation companies as it integrates with Simulink and MatLab great. If you look at the MathsWorks website you can see that NASA and Boeing have used Flight-Gear with there development projects. I have used it, i think it has some good attributes to it, i think it may have a slight edge on X-Plane with Physics and Rendering, but i am not 100% sure as both X-Plane and fight gear are much better than MSFS. FlightGear is open source, so its free!
The Link
http://www.flightgear.org/