Herzog Zwei was like my gateway drug into the RTS gaming world.  When it originally appeared in the early '90s, my friends and I would geek out all night with Herzog, junk food, and over-caffienated soft drinks.  The game was simple enough to pick up quickly but intricate enough to keep you fascinated.  Each unit built had to be given a command, which varied in price depending on what you wanted it to do.  So you had to be smart with your money or else you'd end up with a lot of wasted dough.  In addition, you have to be concerned with ammo and fuel, because each unit only has a limited amount and will stop running/firing if it runs out.  These are the features that make Herzog unique and one of the best RTS games ever!