Deadeye helmet was always my favorite since 4. Shinobi and Helioskrill are my other favorites but Deadeye always wins out. I feel fairly unique with it not seeing too many others with it. Love the concept of super sniper and the look is sleek.
My two favorites are the Achillies and Mark V. The Achillies for the long hours and hard work and the fact that it looks bad -Yoink-! And the Mark V for the resemblance to the original armor worn in Halo: Combat Evolved.