Roller Coater Tycoon is by far one of the best simulator PC game I have ever played.  This game is what it sounds like...You build a theme park.  There are many different environments to choose from(you get more environments as you beat more levels.  The graphics in this game aren't jaw dropping but they are okay.  But this game isn't about graphics, it's about freedom.  It's very challenging to get your guests to actually like your park:)