What do you think, which is the best flight simulator?
What platform?
Is difficult to say. On PC is more a religion war between X-Plane and Prepar3d (formerly MS Flight simulator).
The first has a (little) more realistic flight model based dynamically calculated on the geometry of the aircraft and is updated more frequently with the possibility to access to beta version. It require a little less hardware specifications. It also has a more detailed default world.
The great advantage of P3D is the much bigger list of add-on being compatible with old MS Flight Simulator X. Even if, actually the pace of new add-ons is similar for both simulators, there are some 3rd party aircraft, like those from PMDG and FSLabs, that doesn't exists for XPlane and are simulators inside the simulator.
All this come at cost. P3D costs, in term of 3rd party add-on, much more than XPlane where many good scenery and tools are free.
Generally speaking, if you prefer big airliner, you'l find more in P3D (but you have to pay), while for General Aviation XPlane is better even with no add-on.