There isn't really a clear answer on this with ESO as generally ESO doesn't have tiers of gear and one tier which is best.

It comes down a lot more to what class you are playing and how you want to tank. There are some go to sets which tanks run such as Ebon Armory (Crypt of Hearts I & II).

You can try checking out some builds from Alcast or Woeler (Google the names or click links in my forums signature) for information on tanking builds.

If you're doing normal dungeons you might be able to tank using a medium armour damage based build, if you move onto trials or veteran content then tanking takes on a more specialised role with less damage and more support.

For normal content just make sure you can survive any big hits from the bosses (gets much harder on later DLC dungeons) and slot a taunt skill (Sword and Shield Melee Taunt or Undaunted Range Taunt)

As you progress to harder or veteran based content the general the role of the tank in ESO is to hold aggression of the boss monsters and stack trash monsters (DK Chains Skill, Swarm Mother Set, Silver Leash Skill) for damage dealers to burn down.
They will also be applying buffs (War Horn Skill, Ebon Armory Set) and debuffs (Alkosh Set) to help the damage dealers in the group achieve their full damage.

It's important to note that when it comes to group content in ESO this is when it breaks away from the traditional Elder Scrolls play style and becomes the most like other MMO games where you have a set group (tank, healer, 2x damage dealers) as an expectation.