Flight simulator X. The best flying simulator ever made.
Although it's very detailed (featuring real controls, like the de-icing system, and much more), it lacks of performance. What I mean of that is that even if you play FSX in a very strong computer, the maximum it can get is 50 FPS.
It features an exciting multiplayer mode, it also features voice chat.
FSX remains and will remain a nearly-professional flight simulator. What I mean is that this is one of the best flying materials novice pilots can use to train themselves.
The only bad thing on it though (except the low performance) is that it is hard to control without using a joystick, or a gamepad. This makes the game hard to the novice users (I really had a hard time playing the game when I was a first time user)
Also, I recommend the acceleration pack for those who are familiar to FSX and would lie some more, like new missions, three new aircrafts (including the Boeing F/A 18) and better graphics.