Depends on your taste really, and how you work... If you do well cutting and gluing, halo: reach, If you do well forming, halo 3.
The most customizable option with easy templates would be halo ODST armor, since they are mostly built up of simple shapes.
Halo 3 mk vi is a near second in terms of part numbers. Halo reach may differ in terms of this though.
It's entirely up to you.
Easiest isn't exactly how most people want to do work around here, since the more time done, the better the product.
Most people say mark twice cut once... But I prefer too add: Polish and sharpen your blade after every cut. The sharper the cut, the more visually pleasing.