The weighting of this list is mainly for AQ40 and Naxx.
S Tier: overpowered
A Tier: excellent
B Tier: good
C Tier: barely viable
D Tier: weak
F Tier: meme spec
S Tier
Fury Warrior Tank: This tank is capable of the highest tps in the game and can dps close to a full fury dps warrior when not needed as a tank at all in a fight. The only downside is that it does require you to be hit capped and have a high amount of crit (around 25%) to outperform prot in threat enough to be worth using.
A Tier
Feral Tank Druid: While shunned in retail vanilla, the 1.12 client was very kind to them. They are able to outperform warriors in single target threat unless the warrior has a Thunderfury, do somewhat decent dps in the same gear they use to tank, have the highest hp and armor in the game, and have battle rez/innervate when not tanking. The only thing holding them back is mechanics such as crushing blows, critical strikes, and fears.
Prot Warrior: This is the spec Blizzard designed to be used by the guild MT. They have the best overall mitigation as MT in the game and have solid threat even if it isn’t the best later on. They do fall short in AQ40 and Naxx where higher dps is more than worth taking some extra damage as a fury tank.
B Tier
Fury DPS Warrior: This is on the list because most guilds use fury warriors as OTs for bosses and trash that require more than 3 tanks. They simply slap on tank gear. The reason they are not higher is because they are significantly weaker as tanks than tank speced Warriors and Druids.
C Tier
Arms Tank Warrior: This spec was very common in early vanilla, but 1.12 has not been kind to it. It is inferior to Fury Tank in threat, inferior to Prot in mitigation and threat, and Mortal Strike hogs up a debuff slot as well as the talents being much weaker than both the Fury and Prot trees.
D Tier
Prot Paladin: The problems are they go oom quickly, have terrible single target threat, can’t avoid crushing blows like Warriors and don’t have higher armor/hp to make up for it, and there isn’t much spell power plate gear to be able to generate enough threat. Just wait for TBC Classic if you want to be a tankadin in old content. They are able to produce AoE threat, but so can a Warrior with an engineering shield.
F Tier
Enhancement Tank Shaman: They suffer most of the problems Prot Paladins do except for poor single target threat, but they are still worse because they can’t use plate or a spell that increases their armor significantly and they don’t have AoE threat.