SimCity - Enjoyed both the SNES and DOS versions. Couldn't get much into most of the later efforts for some reason.
RollerCoaster Tycoon 2 - A ton of fun. And I still find the yelling/cheering of the people when they like a ride to be so endearing. The puking, less so.
Railroad Tycoon - Some of the limitations (esp. train/route limit) were frustrating, but it was still a blast. I think this game, Civ1, and Wolfenstein were my introduction to PC gaming.
Aerobiz Supersonic - Played it on the SNES. I still play an emulated version of it every few years. Admittedly it's pretty limited in what you can do and what will get you a win, but it's a lot of fun nonetheless. Some day I'd love to take a shot at making a deeper version of this type of game... something in between the older airline games of yesteryear and the more data-heavy management sims of today.
Rockstar - Text-based sim, but besides the gameplay the wacky "drug use" graphics were fun. ()