Hands down one of the greatest PC games ever made.  I spent many an hour as a child playing the Roller Coaster Tycoon series at my friend Zach's house every summer.  We'd spend weeks building up our favorite parks and being kids not fully understanding the mechanics of the entire game, would struggle to beat each park.  At the age of 24, I still play this game and very much enjoy it.  I understand everything about the game and can build a park to all its strengths with ease.  Chris Sawyer is a genius, and this series is legendary.  While I did enjoy its sequels, the original Roller Coaster Tycoon will always be my favorite.