SubLogic Flight Simulator on the Apple II. Long before MS ganked it. Unless you count the vector-graphics Red Baron arcade game...
'Scuse me while I limp off to get my evening dose of Geritol.
Seriously, IL-2 is about as accessible as so-called "hard-core" sims get, although some don't consider it true hard-core since it's actually a survey sim rather than a study sim-- despite the title.
For true hard-core study sims, they simply don't get any better than the Realism Patched Falcon 4.0. But then I certainly wouldn't describe it as accessible, either. You've got to have your doctorate degree in Third-Party Patchology just get the damned thing running right. And then it takes hours and hours of dedicated practice flights to get to the point that you can successfully pull off a mission-- let alone a full campaign. It's definitely only for the dedicated wingnut.
For sheer fun factor, I far prefer a patched and third-party enhanced European Air War. There's still nothing come close to the feel of attacking a bomber formation the way it's done in that game. It's an absolute blast, and really tests your energy-management skills. Those Luftwaffe pilots who attacked Allied bomber formations late in the war had cojones so big, it required a forklift to get them into the cockpit.