There are two ways to deal with the adds in the heroic Anub'arak 25 man encounter: Have 2 tanks take 2 adds each while each is 12 yards apart, or have 1 tank tank all 4 adds in full unhittable gear.

At the time of this posting, and the help of Rawr to find out the proper avoidance values, I can achieve 97.2038% avoidance. (Updated: current unhittable is now 99.6928) This has been achieved with the standard raid buffs (imp mark, kings, sanc, horn of winter), [39666], [44328], [42999]. This also includes mixology for [39666] for an extra 16 agility, but not [44328] (extra 16 defense rating) as the mixology effects for this elixir requires the ability to actually craft it. I only leveled alchemy over the weekend and the two times I have used Northrend Alchemy Research, I did not learn [44328]. In addition, there has been discussion of possibly using scorpid sting and/or insect swarm on the adds to gain another 3% of being unhittable. While it looks on paper a great way to stay unhittable, and we have 3 hunters and a boomkin keep every add debuffed every 20 seconds, is this worth the lost of 4 people dpsing and switching to debuff the 4 adds enough to outweigh the gain of a tank who has switched to dps? It's hard to say, but I don't want to take that much of a chance if we're going to miss enrage timers.

What does my unhittable gear consist of? http://www.wowhead.com/?profile=15642501 . Yes it is loaded with enchants from BC, but all valid ones. The vaule of wowhead showing what my dodge and parry can have is not correct that it doesn't show the high uptime of [47664] and the high chance that blade ward can proc and stack.
Helm:
16defense/17 dodge is better than 30stam/20 defense

Cloak:
16 defense is better than 22 agi (will be switching this soon)

Chest:
22 defense is better than 10 stats

Bracer:
12 defense is better than 40 stam

glove
20 agility is better than 8 defense or 2%threat/10 parry rating

boot:
16 agility sometimes is better than 8 defense, 22stam, or Tuskarr's Vitality (15 stam +8% run speed) (switching as soon as I can)

Shield:
15 block rating is better than 20 defense or 18 stam

Rings: (enchanters only)
4 stats is better than 30 stam

Gems:
meta:[41396] has more avoidance than [41380] then find 2 sockets to gem with orange or purple gems.
other sockets: Fill other sockets with 20 defense generally unless the socket bonus helps with avoidance like how [39638] has a socket bonus of 6 block rating. Why is Rawr ignoring the other socket bonuses in my shoulders? Probably because dodge will go on DR as my dodge rating is around the point where it can DR, but blocking has no DR.

Upgrades:

To obtain full unhittable for me still requires 4.3962%. (updated:1.9072%) Where are these upgrades. Well oddly enough, there aren't many as I have most of the unhittable items already equipped. There are 2 ways to get the last 4.4% (updated: 1.9%) or so unhittable: grab the top 4 items unhittable items, or grab as many of the "lower" valued unhittable items as i can.
Helm:
.09% [40366] Sapph25

Neck
.67% [44665] Maly25 neck reward
.26% [44660] Maly10 neck reward(has been ticked for a swap, currently equipped)

Shoulder
1.48% [40334] naxx25 gothik or gluth
1.13% [45697] kolo 10 man
.98% [46177] kill yogg25 and get the armor token
.80% [45385] kill torium10 and get the armor token
.6% [48647] heroic armor vendor
.6% [48646] heroic armor vendor
.41% [47698] 45 triumph tokens
.29% [37635](a blue BoP?) 2nd boss in gun'drek
.25% [48637] (trophy)

Cloak
.56% [45322] Coak of the IC 10 BoE (one was found in someone's bank)
.31% Magni's Resolution (not found on wowhead at all yet) heroic 25 man tribue chest
.03% [48675] heroic 10 tribute chest

Chest:
.57% [40279] 25grobbulus or gluth
.52% [40579] naxx25 chest token
.37% [45334] 25auriaya

Bracer:
.12% [47111] heroic 25man twins

Glove: No upgrades

Belt:
.50% [40263] 25patchwerk or gluth
.04% [47076] heroic 25 faction champ

Pants: No upgrades

Boot:
1.76% [47003] heroic 25Lord Jaraxxus
1.37% [37712] h DTK first boss
1.01% [46997] normal 25Lord Jaraxxus
.98% [39717] BoE boots from naxx25 (has been ticketed, now equipped)
.87% [45560] crafted (10 titansteel bars, 12 fire, 6 orbs)
.85% [45988] mirimon10
.84% [47952] heroic 25twins
.43% [47738] 10twins
.07% [40743] 40 valor tokens (may not be worth it as it's so low of an upgrade
.01% [item][/item] ToC5 normal eadric's (may not be worth it)

Ring:
1.31% Signet of the traitor king(not in wowhead) heroic 25 anub'arak
1.25% [45247] BOE XT-25
.68% [47149] BoE 25 anub'arak
.64% [45471] 25 Thorim hardmode
.37% [47955] 10 heroic 10twins
.15% [45326] 25 Auriaya
.10% [40107] naxx25 shared drop
.08% [42643] crafted (maybe not worth the small upgrade)

Trinket:
1.22 [49487] onyxia trinket(currently equipped)

Weapon:
.77% [47506] heroic 25 Tribue chest
.67% [48714] heroic 10 tribue chest
.38% [45442] 25 man xt hard mode
.26% [45876] hodir 10 hard mode
.15% [49495] onyxia 25
.04% [47967] heroic 10 anub'arak

Shield:
2.10% [45877] hard mode hodir10 (JAYEL I NEED MY SHIELD BACK =( )
1.85% [45587] 25 man algalon
1.04% [40400] 25 man kel'thuzad
.11% [37162] neroic nexus

Libram: No upgrades