I'm making another claim, so armorsmiths, get ready to jump in the comments and prove me wrong: This helmet is the highest non-shield armor item in the game. We've covered some nice shields before (and those are pretty much all Armor), but this helm from Zul'Aman has the highest armor value of a non-shield piece of armor that I could find. Update: Yup, commenters have a few pieces with more armor on them (although, yes, PLP only deals with gear on the live realms, so 2.4 gear doesn't count yet). I'll get you next time, readers!

Name: Battleworn Tuskguard (Wowhead, Thottbot, Armory)
Type: Epic Plate Head
Armor: 1355
Abilities:

• +60 Stamina
• Red, Yellow, and Blue sockets, with a socket bonus of +6 Stamina
• Equip: increases your Dodge rating by 23, shield block rating by 40, and block value of your shield by 51
• Word is going around that this is going to change after patch 2.4, however: the shield block rating will be dropped, and the helm will instead raise defense rating directly by 40. That's not confirmed yet, and even if it's been seen on the PTR, there's no guarantee it'll make it to the live realms.
  
• But either way, this is a great tanking helm. In some situations, it even rivals T6, but the best part about it is that you don't even need 25 people to get it. (see below)
• Plus, it does look pretty cool. At first I thought that was a bird of some kind, but instead it's a Troll's face, complete with jutting tusks. If you like the tribal look of the Zul'Aman gear (and I do), you'll like this helm.

How to Get It: It drops from Hex Lord Malacrass, the second to last boss in Zul'Aman. He's not a super nice guy (and he's got a mean Soul Drain), but the good news is that even though he's tough, ZA is still only a 10 man instance. So even if you don't have more than 9 friends to raid with, you can still pick up a great helm like this. It won't be easy, but get him down a few times (drop rate is about 15-20%), be a tank (if your MT is good, he may have gotten something better already, so offtanks have a pretty good chance at this too), and the helm is all yours.

Getting Rid of It: Disenchants into a Void Crystal, and sells to vendors for 7g 30s 2c. You may want to keep it around for Halloween, though -- that's a troll face that only a troll mama could love.