If your stress is caused by the world, your dhikr is "La ilaha illallah", for it brings to mind that there is nothing worth fearing or adoring but Allah.

If your stress is caused by people, your dhikr is "La hawla wala quwwata illa billah" for it brings to mind that people have no power.

If your stress is caused by yourself, your dhikr is "Astaghfirullah" for it wipes away sins which is the top cause of anxiety.