I was gonna address this in my warrior guide which I?m holding off on until server launch, but I might as well do it now since this is a common discussion. For a bit of background information, I?ve maintanked/offtanked all the dungeons up to the last released raid, as well as been in numerous raid groups with other classes tanking, so I have a bit of experience, and not from just theorycrafting.?Scroll down for the tl;dr version below if you want a quick answer.
Although this topic is about main tanks, I feel it?s relevent to talk about the 2 ?Tank? roles, since the ability to off tank should be considered as advantage to main tanks. One is the ?Main Tank? and the other is the ?Off Tank?. The?Main Tank?is the player who will be tanking the boss, and holding aggro until it dies. The?Off Tank?has many different responsibilities. The main thing is to handle boss summons (adds). Other responsibilities includes luring when needed, and holding the boss while the priests/sorcerer?s are ressurecting the Main Tank. (Note that in certain situations, such as some dungeons, a party is capable of taking down adds. Hence, an Off Tank isn?t necessary. However, they become more and more important, the higher you go in difficulty)
Monk: Highest Defense, Second Highest HP. Moderate AoE Control. Good ?Threat? Control.
My Opnion: Monks are great main tanks. Their defensive capabilities make them quite sturdy. Furthermore, they?re capable of generating ?Threat? quite easily. This makes it so that the DPS don?t have to worry too much about stealing aggro. However, unless holding single target adds or a 2nd boss, Monks generally don?t make the best off tank due to a lack of AoE control. I see of ton of monks doing really well in raids.
Beast Master:?Monstrous HP. Moderate AoE Control.
My Opinion: Another great Main tank, capable of reaching monstrous amounts of HP. Morever, with a healing ability, and ability to further increase your survivability during crunch time, they make one of the most reliable single target tank in the game. Their threat generation is low however, and DPS needs to becareful about stealing aggro from the bear, which is an issue sometimes. They?re also in the same boat as Monks when it comes to Offtanking in the sense that they dont have a consistent aoe skill.
Knight:?Decent HP, Decent Defense, Good AoE Control, Good Threat Control.
My Opinion: These guys are the ?Go-To? tanks. They?re good all around, and they get an extra piece of equipment for more stats. As far as AoE control is considered, they?re the second best. Main tank or Off Tank, You can?t go wrong with a knight, and I see tons of them.
Warriors: Highest Parry Potential, Excellent AoE Control, Decent Defensive Capabilities.
My Opinion: Warriors get a lot of hate as being a ?Half Melee DPS/Half Tank? class that isn?t capable of Main Tanking. However, I can tell you from this experience that it isn?t true. They?re the best at Off Tanking, but they?re also capable of Main Tanking. The only issue is that it takes warriors a lot longer than the other classes to become strong main tanks, hence why they?ve adopted the stigma that they?re incapable.
Knight Tank vs Warrior Tank
This question?ll be asked eventually, so I thought I?d address this as well. Now, its generally accepted that warriors asre the better Off Tank (Higher DPS to kill mobs quickly, as well as their aoe skill Brandish Storm allowing them to move while casting, whereas Knight?s AoE Skill GrandCross prevents you from moving while its casting). The main reason why Knights are considered ?Better? tanks, is the fact that they?re great tanks right from the start. As soon as you hit 50, you can go ahead and tank as you please, due to the extra stats from the shield, as well as its skillset. However, warriors not only have less stats, but rely on Parry as the key difference maker. This makes it so that Warriors aren?t really reliable tanks UNTIL late game (Where their HP can catch up, and their parry rate can reach 100%). However, once they do reach that point, one can make an argument that warrior tanks have the potential to be the -strongest- tanks in the game.
So what do I think is the ?Better? Tank? I say Knights, because they?re easy to pick up and can start?safely?tanking right away, hence why they?re the most common and loved tanking class. While warriors have the potential to be the ?Best? tank, it takes a ton of work, which many people don?t have the patience for. Hence, there are very few Main Tank Capable warriors, but those few are heavily desired in parties. Monks/BMs are the better Single Target Tanks, but lack the AoE needed to handle adds.