Halo Reach in general.
I felt the design of Halo Reach's main armour to be the most "functional" of all the Halo games. Aside from being aesthetically pleasing, the attachments made it feel like armour that would not only protect the spartan, but have tactical advantages in the battlefield. While not as sleek or recognizable as Gen 2 MK V or VI Mjolnir, it seemed more suited for field engagements. Jun's camo and sniper bullets, Kat's comm links and camera (being the intelligence agent), and the various ammo pouches like Recon or the softcase.
I found the armour featured in Halo 4 to be too show-boaty by comparison. Designed as opposed to manufactured, if that makes any sense. Lots of seemingly pointless plastic pieces jutting out here and there that made the designs less simple/sleek, and also less functional. There are exceptions of course, and I genuinely liked several pieces of Halo 4 armour, however I was unimpressed overall. Halo 5 looks like it's fixing many of the problems I had with its predecessor.
ODST - Halo 3: ODST's
ODST helmet remains my favourite out of every other armour piece. Something about the design resonates with me, and the visor colour compliments it perfectly. Unfortunately I haven't liked it in any game since, where either the design wasn't quite the same (Halo 3-4) or the lack of proper visor colour threw it off (Halo: Reach).
Halo 5's variation of the helmet looks like a return to form, one that I'm very happy for.
Commando (UA/FC Attachment) - I love the Mjolnir MK V and MK VI helmets, however Commando from Halo reach takes the cake for me. While I wasn't necessarily a fan of Carter, I was set on the Helmet design once I saw a particular piece of concept art
. It is, to me, the quintessential Halo Helmet, looking like a mixture of the ODST and Mjolnir MKV designs.