Well... the first one I have played is Bruce Artwick's original flight simulator / 3d graphics demo on an Apple II. I think I have it on a disk in a lock-up somewhere in the UK, if it hasn't got too damp by now (and if the owner of the lock-up hasn't thrown it out -- Toby, you listening?)...
But the first one I played was his Flight Simulator II on the Apple II. On a green screen, it was kind of difficult to see the difference between land and sky at times, and playing using paddles rather than a joystick made life a little more complicated too. But I guess I was hooked anyway.
I remember one on the Spectrum from the early '80s, too.
Over the years since then I've had or played (off the top of my head) FS3, FS4, A10 (that was great!), Falcon 2 (mainly pink CGA graphics!), Falcon 3 (I can hear the "Fox two Mike!" now... but the game never really lived up to its potential, for me), Falcon 4, Il-2, one called something like "F18 Interceptor" (or was it F-22?) from the early '90s, very much game over sim... bought the Tornado one way back too but never learnt to play it (that, like Falcon 4, really was a simulator more than a game).
Most fun - probably A10 and now CFS3. I like Il-2's tutorial mode and "feel", but the graphics are a letdown compared to CFS3. Falcon 4 promises much when I get round to learning it.
