The Atronach is the most popular Tank Mundus Stone.
For virtually all situations you’ll benefit much more from using The Atronach than any other Mundus Stone as an ESO Tank. This is because you typically always prioritise Magicka based skills and most of your self healing and crowd control skills are Magicka costing.
Also as a Tank while you are blocking, you are still able to regenerate Magicka. On some Tank Classes you may also have Magicka based Tank skills that return Stamina while blocking which makes your Magicka even more important.
For Tanking, the most essential thing you need to survive is sustain. Especially to begin if you don’t have the Mages Guild skill Balance when Magicka Recovery is one of your only methods of gaining Magicka sustain.
If you have Magicka then you can heal, you can shield, you can cast skills and you can protect yourself. Since you are unable to regenerate Stamina while blocking and Health Recovery isn’t extremely beneficial, this leaves on Magicka Regeneration as the only useful sustain based Mundus Stone that is available.
Another Mundus Stone option is The Steed which is good for both Cloudrest and Asylum Sanctorium when you combine it with Divines gear to increase both your movement speed and your Health Recovery. Other than those two locations is not really that vital anywhere else.
The last useful Mundus Stone is The Lady. It is not essential to maximise your resistances to be good at Tanking in ESO, having 25k+ will allow you to complete all content in the game comfortably, and don’t forget that resistances are capped at 33k so make sure you don’t ever go over that value when you are buffed with Major and Minor Resolve. When doing burn strategies, high risk strategies, or dealing with damage while spending large portions of time unblocked, this is when utilising more resistances can be useful.
Aside from these, no other Mundus Stone is really useful for Tanking. Max stat Mundus Stones are simply irrelevant since the Flame of Ambitions patch where we got a stats boost when minimum stats were introduced, we can achieve extremely high max stats without the minor increase from using a stat Mundus.
For more Tank tips and information check out The Tank Club Beginner Guide & The Tank Club Discord