Rollercoaster Tycoon...

I was so f*king addicted to this game, and so were many others. I think the fact why this style's so popular, is that it gives the player control. You can create your own rides, and scenery, like a level editor, only in realtime with strict limits (money, time, obstacles).

I'm not sure how you might implement this into another type of game, but this game scored major...