I'm torn between the Mk 5 and ODST helmets
I don the Mark V with the full setup because I like it. But what I feel like most did NOT like was the customizations to each helmet/armor and a lot of the helmets/armor themselves. This is because this grade of armor was meant for Spartan-III's and Spartan-III's were able to mix and match their armor where most Spartan-II's (save for Jorge) wore straight down sets; with the extent of mix and match being, say the CQB helmet with EVA grade shoulder pads, a recon chest piece, etc. The third generation of Spartans were able to and allowed to fully configure their armor to their preference, even having two different shoulder pads.
Unfortunately none of the helmets in Reach are appealing to me. For the longest time I wore the grenadier variant but have recently switched to the security variant with the attachments. I do really like the look of the haunted helmet and it's a goal to wear it one day hopefully.