Tanky can both relate to the amount of sheer defenses against physical and magical attacks, pvp gems and crowd control skills.
Any combination of these can be utilised. Cloth gear with physical shield complemented, or plate shield with magical shield.
Paired up with Witchcraft or Occultism, and Auramancy for mobility, and gemmed for resilience and toughness can produce great results.
That means there is no single strongest tank, it fully depends on the amount of damage you are prepared to turn into defences.