I would have to say that the best would be Microsoft Flight Simulator 2004. This is because it comes with a huge amount of support material that will get you up and running quickly. It uses real navaids and the flight model for the default JetRanger isn't bad. You can also download many other aircraft models off the net, or purchase them, that emulate other cockpits such as AS350, UH-1, etc. You can also purchase printed manuals to learn how to run the instruments better in the program itself.
Simulating spatial disorientation
To simulate spatial disorientation, change the forward view to be one of the side views. This will force you to focus on the instruments much more intently, and will trick your brain to produce that weird feeling you get when your inner ear wants you to fly in a different direction than what your instruments are telling you.
Flight Controls
I would recommend spending the money to get a full size set of flight controls. A $50 joystick with handle-twist anti-torque and dial throttle/collective just doesn't cut it. You can also get a lot more serious and get a full radio stack, and even GPS units that are fully integrated with the sim program.
Having fun
There are several enjoyable combat simulators that provide a nice break from staring at instruments. One I can highly recommend is slightly old but very playable and addictive - Enemy Engaged (Comanche vs. Hokum).
Hope that helps.
Cheers,
SLC.