Spore

During spring break in Colorado my three nephews have been showing me some of their favorite games, including Spore, which we all agree is really amazing.  Rated 10+, it's a complex, visually incredible, multi-leveled game of evolution and adaptation.  The game starts with single-celled organisms and culminates with intergalactic space travel.  One of the most impressive aspects of Spore is the design element;  you get to design everything from creatures to vehicles to town halls to spaceships.  And there are hundreds of features and colors to choose from...every game can be different. 
There are five stages to progress through beginning with Cell and moving to Creature, Tribal, Civilization, and finally to Space.  At each stage there is an "economic" component;  you spend DNA to make your creature more evolved, for example, or look for food which enables you to help out your tribe.  Once the civilizations have evolved, there are Spice Geysers to seek out and mine, and Spore Bucks to spend and earn.

You learn as you play this game that you can change your mind about something, go back and adapt your creature or try a new strategy.  Things are flexible and always evolving, which is what makes the game challenging and fun.
One part of Spore that I find interesting is that a player can decide how they want to "behave" in any particular game.  The creature you design can be an herbivore, a carnivore, or an omnivore, and this determines the level of "violence" in a game.  Or, later on, you can decide if you will trade with other cities and planets, or try to destroy them.  There isn't an obvious advantage either way, though my nephew Miles told me if you go around destroying all the time "people won't like you very much."
Eventually there are the agressive Grox to deal with, creatures who live at the center of a galaxy and must be overcome to finally end the game.  However, my nephews have had Spore for almost a year and haven't come to that point yet.  There are so many cool creatures, structures, and space ships to design, this game never gets boring, or old, or even finished!