The best replacement for fsx is fs9.

The best alternatives to fsx are rated as following:
★★★★★  fs9
★★★★  fs2004
★★★★  x plane
★★  p3d
★★  xplane
★★  flight
★★  fsx deluxe
★★  fsx gold
★★  prepar3d
★  p3d v2
★  flightgear
★  x plane 9
★  ms flight
★  dcs
★  prepar3d 2.0
★  xplane 10
  x-plane 9
  fs9 better
  fs2004 better
  x-plane
  microsoft flight
  prepar3d v2
  x plane 10
  xplane9
  prepar3d performance
  flightsimulatorplus
  prepared 3d
  fs 2004
  x-plane 10

Your alternative for fsx is not listed? Suggest!
Speaking about fsx, one probably shouldn't forget about foreflight.

Other search terms related to fsx and their ratings are:
★★★★★ foreflight
★★★★★ oculus rift
★★★★★ dx10
★★★ multiple monitors
You have in mind a companion for fsx and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of fsx in all years. Click on graph for more.
fsx
fs9
fs2004