Various. with zombie customs I usually start with the head and use otherwise useless body parts. Or body parts that I have duplicates of.

The undead, growing in numbers, spreading the infection one victim at a time.

These are various examples of zombie customs I've made over the past year or so.
In my End of Days Dio-Story zombies share center stage with characters from GI Joe. So it's important to me to have many styles and zombies from all walks of life.
I have to say there's no custom that's more satisfying to make, for me, than a zombie- that's mainly because you can't really mess it up. The more you dremel and dirty up when you paint to more character the zombie gets. I think everyone should try making one at some point if you customize.