If you want one that's actually fun for more than 10 minutes, get Hawx.
Otherwise MS Flight sim X is probably the best as far as true simulators. I've heard X-plane is pretty good as well but I never tried it because it looks like a rip-off of MS flight sim.
Both have a huge modding community. The mods are really the only way to make it much fun IMO. I remember playing online with a friend, flying an AN-225 upside-down through a canyon. That fun lasted about 10 minutes, then we got bored and played counterstrike.
It would probably be far more fun with these instead of a regular ****ty joystick:
X65-F Combat Control System
