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.