For me, it would definitely be Sid Meier's Alpha Centauri. I played this game for at least two years. I never got into the expansion, but damn, this was, IMO, one of the greatest turn-based games ever conceived. The gameplay itself was also extremely intense especially given that it was entirely turn-based.
Unfortunately, even Rise of Nations, while better in general, doesn't offer the complexity of SMAC.
Another game that I played that no one else really did was Jane's Fleet Command. This was a realtime strategy game with turn-based principles. Unfortunately, when I installed mods for the game (the base game was entirely too simplistic and in some cases, factually incorrect), the program itself was plagued with a glitch that would drop be back to the desktop with no warning whatsoever, as soon as the game had a certain number objects to keep track of. Thus it was never possible to have what I would think would be a true naval simulation - multiple CVBGs with several airwings in the air, with several dozen cruise missiles and SAMs all on certain trajectories. This limitation really limited the playability, but that didn't change my opinion that it as a good game once the mods were installed.
Unfortunately, even Rise of Nations, while better in general, doesn't offer the complexity of SMAC.
Another game that I played that no one else really did was Jane's Fleet Command. This was a realtime strategy game with turn-based principles. Unfortunately, when I installed mods for the game (the base game was entirely too simplistic and in some cases, factually incorrect), the program itself was plagued with a glitch that would drop be back to the desktop with no warning whatsoever, as soon as the game had a certain number objects to keep track of. Thus it was never possible to have what I would think would be a true naval simulation - multiple CVBGs with several airwings in the air, with several dozen cruise missiles and SAMs all on certain trajectories. This limitation really limited the playability, but that didn't change my opinion that it as a good game once the mods were installed.