My top three are:
1. HAZOP (shame there isn't a good file for that helmet yet)
2. ODST (However, in halo 3 it doesn't really flow with the armor...)
3. Operator, UA (Up armored) version (I also wished that there was a file for this, I would love to make one without the Camera, or UA. I think it would look really nice...)
Shame only one of them has a good file here.