I did not write this. Ralthos of Cenarious wrote this on Maintankadin. I'm copying it here to my Blog for future reference.
- Solid Star of Elune (12 stamina, Blue): This gem gives the largest bonus to effective health of all gem options prior to the BT/Hyjal gems, which are 15 stamina each and ridiculously expensive to buy. If you are uncrushable by a solid margin, this will give you the most survivability of any gem option.
- Enduring Talasite (4 Defense Rating and 6 Stamina, Blue/Yellow): The best yellow hybrid available, giving both a modicum of avoidance and stamina. This is also a solid gem choice for filling the defense gap in the 2.3 badge/ZA gear. This gem is best used to complete socket bonuses in gear, especially gear that has a stamina socket bonus. Completing a socket bonus for 6 stamina on a 3 socket piece is worth it most of the time, even if you lose 6 stamina for a R/B/Y socket color set (as is the case with a number of good epic tanking pieces). The defense rating is worth ~1.66 defense, which is adjusted based on your total defense, making it either .16 avoidance or .32, based on how much defense rating you have.
- Glowing Nightseye (5 spell damage and 6 stamina, Red/Blue): If you feel slightly low on spell power and have a red socket to fill, this is a good option. Generally, however, this is inferior to Solid Star given that a weapon can give sufficient spell power to hold threat for a long time in terms of raid progression. As with the Enduring Talasite, this is a choice gem for completing socket bonuses.
- Shifting Nightseye (4 agility and 6 stamina, Red/Blue): Another red gem option, giving slightly more avoidance in lieu of threat. The agility is worth .16 dodge. -Subtle Living Ruby (8 dodge rating, Red): The best straight red gem available for conversion. If you're short avoidance and need a red gem, this is a good gem to choose. Each gem is worth .42 dodge.
- Thick Dawnstone (8 defense rating, Yellow): The only straight yellow gem worth considering whatsoever. The defense rating is equivalent to ~3.33 Defense, which rounds based on your total defense rating. It is approximately either .48 or .64 avoidance.
General Gem Theory As Joanadark wrote at Tankspot, gear is more efficiently itemized into mitigation/avoidance/stamina than spell damage, hit, spell hit, etc. In that regard, Solid Star of Elune is the most powerful gem available for overall stats. Consumables are more effective for threat than survival, though, so if you're having issues with that, then look at consumables for threat instead of spell damage on your gear slots. Your weapon will probably be a threat weapon due to the stats allocation anyway, so gemming threat is usually not as powerful in general. The general rule here, though, is that if you lack uncrushability, gem to it, while if you have a solid basis of uncrushability, gem to Stamina.
Helm: Glyph of the Defender, Heavy Knothide Armor Kit, Glyph of Power
Of these, Glyph of the Defender is the only worthwhile one. GoP is hardly worth considering for threat compared to the ~2% avoidance from Defender. Similarly, 10 stamina from the armor kit is substandard by comparison.
Shoulder: Inscription of Warding/Greater, Inscription of the Knight/Greater, Heavy Knothide Armor kit Obviously, the greater inscriptions are better than the normal ones. Like the Glyph of the Defender, they're also better than the 10 stamina from Heavy Knothide Armor Kit. For that reason, work on your aldor/scryer rep.
Cloak: 120 Armor, 12 dodge rating, 12 agility
This is the hardest choice among enchants, in my opinion. Armor is the basis for effective health, which makes it a solid choice on any cloak. Agility is a DPS enchant, really, as established above in the gems section. Dodge is a good choice for avoidance, particularly if you're under the uncrushable benchmark. Personally, I agree with Ciderhelm's evaluation: enchant to the strength of the cloak. If the cloak is primarily for effective health, go armor. If it's for avoidance, go dodge. Devilshark should, for that reason, usually go dodge, while Slikk's can go either way, Gilded Thorium to armor, and so on.
Chest: Exceptional Health (150), Heavy Knothide Armor Kit
This one is an obvious choice for the 150 health enchant. You can't make the stamina armor kit compare. Noteworthy alternatives, though not nearly as good, are 6 to stats (60 health, 90 mana, .24 dodge) and 15 Resilience (crit reduction, but you should be getting that through defense unless it's a resistance set, where you need the health anyway).
Bracers: 12 stamina (Fortitude), 12 defense (Major Defense), 15 spell damage (Spellpower)
12 defense is ~.6 total avoidance, while 12 stamina is 120 hitpoints before talents and kings. I prefer 12 stamina. 15 spell damage might be ok for a threat set, but is largely unnecessary as an enchant. As mentioned above, spell damage is best gained through your weapon and consumables, while mitigation and effective health are best achieved through your armor/enchants/gems. 12 defense is great if you're low on defense overall.
Gloves: Heavy Knothide Armor Kit, 20 spell damage (Major Spellpower), 15 agility (Superior Agility), 2% Threat (Threat), 240 Armor (Glove Reinforcements)
This is another very difficult piece to choose an enchant for. 15 agility = .6 dodge, which is a solid choice for the slot, while Heavy Knothide is 10 stamina. In the case of this slot, where the spell power enchant is so potent compared to avoidance stats, it's also a reasonable contender. Lastly, if you reach a certain (fairly high) spell power threshold, Threat does extremely well. Personally, I use Heavy Knothide. As with the cloaks, though, I recommend enchanting to the strength of the gear. Many tankadins swear by Major Spellpower and, truth be told, it is a very potent option. 240 Armor is powerful, but it sees poor returns until your HP is quite high. It's a great consideration for T5-T6 quality gear.
Legs: Clefthoof/Nethercleft, Mystic/Runic Spellthread
Nethercleft is the top effective health leg patch option. Honestly, nothing else holds a candle to it, in my opinion. A fair number of tankadins consider use of spellthread instead, however, for the increased threat. I'm not so anxious to pass up .48 dodge and 200 health unbuffed for 35 spell damage, however. The lower versions (Mystic/Clefthoof) are fine options for transitional gear to save cash, but when you do get a substantial piece that will last for a long time (badge leggings, for instance), Nethercleft is THE way to go.
Boots: Boar's Speed, Fortitude
These two enchants are pretty much the only way to go as far as tanking is concerned. Personally, I value mobility, which makes Boar's Speed very attractive if you didn't already sacrifice 3 talents for Pursuit of Justice. Since PoJ doesn't stack with Boar's Speed, Fortitude is a superior option for those who spec for it. Mobility is something that Paladins struggle with, so having any little boost is quite significant, especially when it only costs 3 stamina. Rings: 12 spell power, 4 stats While an obvious choice for any Warrior or Druid enchanter, a Paladin enchanter is sort of up in the air on this. As far as potency, 12 spell power is pretty strong. I prefer 4 stats as an option, as it gives more effective health. 4 stats is worth .16 dodge, 40 health, and 60 mana per ring, which I consider to be quite powerful. As before, enchant to the strengths of the gear, however. If you're using a spell power ring, use a spell power enchant every time.
Weapon: Mongoose, Major Agility, Major Spellpower, Battlemaster This definitely falls into the category of enchanting to gear strength. I carry a mitigation weapon (Sun Eater) with Mongoose and a spell power weapon (Gavel of Unearthed Secrets) with Major Spellpower. Major Agility is a more consistent option for tanking than Mongoose, giving a static .8% dodge, but seeing Mongoose proc an extra 4.8% dodge for 15 seconds every minute is quite rewarding. Essentially, it's like popping a trinket every minute. Spellpower is a stronger general enchant, giving a significant boost to threat, which is better for nearly 90% of the content you'll be doing. Sun Eater gets pulled out for certain extreme cases, such as Magtheridon's final add. Everything else gets the Gavel. Battlemaster is mentioned because, on certain encounters with AoE damage, the proc can generate a significant amount of threat. It is not a great enchant, by any means, however. Spellpower will generate more threat per second overall than the 1 proc per minute you'd expect out of Battlemaster.
Shield: 18 Block Value (Tough Shield), 18 stamina (Major Stamina), 15 Block Rating (Shield Block), Felsteel Shield Spike This enchant is incredibly dependent on the shield you are enchanting and the gear level that you're at. 18 stamina is a great catch all, particularly for something like Crest of Sha'tar, where you can stack the stamina pretty high. Block Value is incredibly powerful on block value shields, like the Bulwark of the Amani Empire or the Arena PvP shields. Block Rating is powerful if you're not yet uncrushable, as it's nearly 2% total block, a dramatic increase for any starter tankadin. Finally, the Shield Spike is good for a damage shield, like the Petrified Lichen Guard, which you're unlikely to use in a raid setting. Generally, I prefer 18 Block Value because I'm a BV whore, but you should have a shield with at least 2 out of the 4 options on your person all the times. Block Value and Major Stamina are preferred, but not everyone has uncrushable as they're walking into Karazhan. Fortunately, all of these enchants are fairly inexpensive, save for the Shield Spike, which is more of a Warrior thing for OTing than anything. Lastly, yet again, enchant the shield to the strength of the shield. You will find fewer returns from Tough Shield on an Aldori Legacy Defender than you will on Shield of Impenetrable Darkness compared to stamina, while a Bulwark of the Amani Empire needs the extra Block Value from Tough Shield.