Various people have used various things. I think the most effective build I've seen had a hockey helmet for the base. My instinct would be find whatever has the least padding, as I wouldn't want a bulky helmet as a base, as you need to build and add, which may make your head look oversized. Good luck!
Gamer - Artist - Cosplay n00b
Works in Progress Gaige - 75% done (on hold) Gecko Moria - 90% done, props need to be finished