Space Shuttle Mission is an excellent simulation of what the crew do inside the shuttle, ie, flipping switches. But it is not a realistic orbital flight simulator and has little to no orbital mechanics simulation, it is all on rails with pre-designed flights, whichis realistic).
Orbiter is a good general purpose simulator, if you stick to realistic vessels. Not as much cockpit realism as ssm2007 but better flight (you can choose your own orbits and perform unscripted rendevous and docking manouvers for instance)
There are much more accurate orbital mechanics simulators and calculators available than the above, but these generally don't include any kind of flying or piloting aspect.
I have been playing a little with ZOOM (a trajectory and flight simulator) but haven't done enough with it yet to say if its good or not..
Finally, kerbal space program isn't very accurate but its quite good at simulating an engineering mindset and space program development approach.