Mine is my totally off grid self-built solar house and Earthship!!! Next or first, would be my only child--the biggest environmental thing I did was have only one child. Then after them the LCD TV, LCD Energy Star computer, and composters (with composters being the real greenest by replenishing the soil). Of course all our lights are CFs and flashlights LED. Our low water and power use Staber washing machine is very green and runs on the modified square wave inverter power (15 gal. and 135 watt/hours). Then would be our hybrid vehicle, that we drive an average of 10 miles per day.
Of course, there are some "eco-friendly" trinket devices and products that don't amount to a hill of beans against the eco-problems of now and the future. I hope you don't mean garbage like that!
That's a tough one. Just how eco-friendly are we talking here?