SimAnt is the oft-overlooked early Maxis game that I still think is their all-time best work. To this day, there's nothing quite like it. The concept is pretty simple, involving taking control of a black ant colony warring with the red ants, and slowly taking over the yard. The execution is surprisingly deep, and while I wouldn't say the game is hard, you have to be smart about planning out your colony to survive. It's one of the first truly addictive games I ever played and I'd love for more people to get a chance to experience it.