I have four.
First, in terms of time frame, is Golden Wings, covering the period from the beginning of flight to the end of WW2, depending on what part of the world. In various parts of the US it’s the twenties or thirties, except WW2 in a few spots like Muroc. In the UK it’s the twenties, thirties, or WW2, depending on where you are, and in the Pacific it’s 1941 in Hawaii but WW2 is raging further west.
Next comes FS1954, A Half Century of Flight, covering the period from the end of the war through the fifties. It’s heavily developed in the US and UK, fairly so elsewhere. Lately it’s been my primary area of interest but the Golden Age still appeals strongly.
Both of those are very heavily developed with period correct scenery and AI.
After that comes FsJetAge, covering most of the sixties. It’s almost as developed as the others in the US and UK, not so much elsewhere. I’m mainly interested in the Golden Age and the Classic Era, with just enough interest in jets that are just a little too modern to fit into FS54, so I have JetAge with its early sixties-ish AI and static models on the airports.
Finally there’s Fs2004, developed only in my local region of western and southern New England, and that I’ve never actually flown in that I can remember, which is why it’s development stalled and will possibly never resume. I started it so I’d have a sim to fly modern planes in, but I have little interest in modern planes and because I spend all my limited hobby time fiddling with models, scenery and AI, I never even get time to fly the planes I’m more interested in, in the time periods that appeal to me more. If I have to install from scratch on a new computer I probably won’t bother with this one.