Most of my teenage years were spent playing Sim City 4 Deluxe Edition. That was and is still my favorite game ever. I still play it frequently today, though being that I'm on a different computer now I don't have all of my old cities. The custom content community has really given that game the lifespan it has had.
I also used to frequently play Chris Sawyer's Locomotion, Sim City 3000, Sim City 2000, The Sims, The Sims 2, Roller Coaster Tycoon, and Roller Coaster Tycoon 2. Another sim game that I think was underrated is The Movies, but because it only runs on Windows XP, 2000, ME, and 98, I haven't played it as much since upgrading to Vista and later to 7. The Movies will quite simply not run due to both the game and Windows saying it will have issues on 7 and Vista.
There was a time when this genre was almost over-saturated with games, especially all those "Tycoon" titles that used the popularity of Roller Coaster Tycoon to try to boost their sales, but now it's very thin in terms of total games.