For many years, this was the MAAM Sim R4D for me.
Now, for me, A2a products like the B17g are on that throne, as well as Vertigos SBD.
Generally, "I'm there" when the flight model is not over simplified, e.g. when you really have to stay inside the typical performance envelope of the aircraft and things like prop torque and aerodynamic effects are properly modeled, and the mechanical systems are simulated, also with a damage profile (engines!).
Another source of immersion for me is the noise, which is way too simple in most FS aircraft. A2A does a good job here, with a diffferent wind noise when you open the cockpit for example, and all kinds of mechanical sounds from the engine(s) and systems.
Another effect I miss from the old days of flying Janes WW2 Fighters is the sound effect when pulling G's, remember that? The more you pulled, the flatter the sound got, like reducing bass and treble from the sound spectrum. One also heard noises the airframe made under stress, although I don't know if this is realistic. I've been in a real T-6 doing aerobatics and I did not hear any noises like that, despite pulling G's that almost blacked me out.
Cheers,
Mark