Master Chief, Spartan 117
This has been my big project for the past few months. He was built to be displayed at Bricks By the Bay last month, and I may or may not have stayed up until 2 AM finishing him the night before the convention :)
I don't usually build this big, though; I learned a lot about structural and balancing issues, and I also got my first experience with BrickLink ordering all of that green.
The model is fully articulated and 18" high when fully standing up.