Some more thoughts here...
I have been using the Microsoft Flight Simulator line since the 1990's. Have gone through the DOS releases and FS95, FS98, FS2000, FS2002, and so on, but when FS2004 hit my computer all those years ago, all those titles were dropped into a receptacle and forgotten about. They are setting in storage with other big box games i have collected over the years.
That said my FS9 install has been so heavily modded now that it went from that small install package to well over eating up 120 GB, all that space has been devoted to bringing it back to the Golden Age and the early 1960's when jetliners were just getting their landing gear down on the runways.
I have other flight sims i like installed on my "workstation", but they are no where as heavily modded. I keep Microsoft Combat Flight Simulator 2 onboard because i love fighting in those old planes from World War One up to Korea. That is another one of those Flight Sims i think that will remain a timeless classic.
Then we come to Microsoft Flight Simulator X Deluxe and Acceleration. I keep this sim in the system for a couple of reasons. I use it as my "modern" era install (though FS9, still has an install for that on my other box), and the other, is because my late wife brought it home to me one day, and i can't forget about the smile she had on her face when she reached into her bag and pulled it out, and saying "honey, I got that new flight sim for you!". God, that memory brings tears to my eyes even now. (God, I miss her more then anything i can explain). I have modded it as well, but keeping it "modern". It still works well, and i think it might become a timeless sim like its older brother.
Now i have recently installed the copy of Lockheed's Prepar3dv4 Professional Plus version on my "workstation" and well to be honest i am not as impressed with it as i thought I would be. Good graphics, good use of resources on the pc, but i don't see it really offering me anything I don't already have with the other sims i have listed above. Maybe in time...hell, I don't know? We will see if it grows on me like the other sims have. The one thing Prepar3d has going for it, is the fact that it still is part of the Microsoft line of flight sims in everything but name, and some parts of our community are starting to embrace it as such! I have no doubt this will eventually lead to more growth of this part of the community.
I enjoy the Microsoft line of Flight Sims. Hell my hobby grew up around them, but with all that i said in this post the most important thing is for some reason, I keep coming back to FS2004:ACOF. Can't say why, but I do!!!