Halo 3. Halo 3 is what got me into programming, the things you could do, the things you could make. I was simply amazed with how you could use a cone as a weapon, or make things break down as you damaged them.
Halo 3, simply because of the mechanics that it added with the guns, and forge. Granted, halo 3 would not be the greatest (in my opinion) of the three games. I would get so angry that I couldnt fly all about last resort.