For Main Tanks the top class is either Dragonknight or Arcanist. They are both equally able to be the top class but for different reasons.
In most cases the Dragonknight is always useful for a Main Tank spot in a group for the Stagger buff and if needed the other buffs too. However, if your group decides they don’t want to use the gear sets Roar of Alkosh or Crimson Oath’s Rive and they want an Arcanist, then you MUST be an Arcanist Main Tank.
The unique group buff of an Arcanist is a taunt skill that debuffs the enemy by removing 2200 armor, so if your group want this then you have no choice other than to be the Main Tank, combined with the Arcanist offering a bunch of other extremely useful buffs they can very quickly become the top Main Tank. On the flip side, you can provide all of the Arcanists buffs with someone else playing one so they are not essential if you don’t use the armor pen debuff.
Sorcerers are in at 3rd thanks to their strong defensive capabilities and the fact that they can now provide a full group synergy of Major Berserk and you’ll need 2 Sorcs in a group, providing an opportunity for a Sorc Main Tank to come into the mix. They have a couple of other groups buffs too but the big thing is they are easy to play, one of the easiest to sustain and their survival is great, only beaten by a Necromancer when dealing with DoT damage.
The remaining classes are tricky to rank. Necros are great for DoT damage reduction and many groups will switch the Main Tank and Off Tank around for certain fights for that reason. This doesn’t mean Necro Main Tank is a mainstay for the role though.
Nightblades actually make incredible Main Tanks and they are probably the best “Tank” in ESO as they can buff themselves up a lot and debuff the enemy more than any other class but the benefits they offer a group are severely lacking.
Warden Tanks, I personally prefer to play as Off Tanks because they really have poor quality passives and are not great defensively but they do offer some nice group benefits, these are typically covered by a Warden Healer however.