Pro Flight Simulator is a payware version of the freeware Flightgear simulator. With the latter, you get what you pay for; with the former, you get less than you pay for.
For real-world aircraft in normal flight, Microsoft Flight Simulator is the most realistic. For experimental aircraft, theoretical aircraft, and aircraft in aerobatic or unusual flight regimes, X-Plane may be slightly more realistic, although it is less realistic for normal flight in normal planes. MSFS has the best selection of third-party add-ons and the best visuals. X-Plane will run not only on Windows but on some other platforms as well. MSFS is no longer in development, which means that no updates are forthcoming (but neither will it become obsolete). X-Plane is in active development.
Perhaps the best configuration is MSFS with all the add-on trimmings: add-on aircraft, add-on scenery, add-on weather, flight controls such as a yoke, throttle, and rudder pedals, and perhaps even multiple monitors and/or TrackIR. You can spend anywhere from $19 to $19,000 on it, depending on how far you wish to go.