In my previous life this was the last Flight Sim I used (a friend of mine also had FS98 but I didn´t have a PC back then):
In the actual incarnation this was the first flight sim I was using 
For some reasons FS2004 is still considered as one of the best.
But FSX had more and waaay better third party support, and all the awesome PMDG´s and Captain Sim´s (yes Captain Sim was good and not Captain Scam back in the days!), and Carenados and A2A Accusims and Rex and OrbX, and so much more.
Flight Simulator is a long franchise, legends say that it was already here back in 1995.