The flight sim I spent an insane amount of time on.
Second only to Falcon 3.0.
Gunship 2000 - the best helicopter combat simulation ever? It's 35 years old now, so I guess the answer is no, but honestly, I’ve never come across a better one.
Very steep learning curve, massive manual (in typical MicroProse fashion), and excellent gameplay. If you invested enough time in it, it would reward you a hundredfold.