The MJONIR Powered Ass Salt Armour/2+2=10 Mark V is one of the best helmets. It is the one you use in Halo 1 unless you're a modder.
I think the Mark V is a pretty cool guy, eh has recharging energy shields and doesn't afraid of anything. It has a neat visor, neat shoulder pads, neat chest, neat boots but watch out for Grunts, they might bight your kneecaps off!
In Halo 3, it only had the Helmet (EOD stole the chest) but the helmet was sick, in Reach it looks worse than Pizaa, so it broke the world record of being the only Helmet that ended up as shit in Reach.