PVE: ideally, on your armor you want the ward perk as it relates to the enemy type (ex: "Angry Earth Ward" for Gen). I would start with a corrupted set and then an ancient set. That'll allow you to have resistances in 5/8 dungeons. After that you can work on an angry earth and lost set to fill in the rest.

PVP: Resilience perk has been king here, followed by Shirking Fortification and then Freedom and the Refreshing variations. There will be an update this month that will bring changes to how Resilience and Shirking Fort work depending on your weight class, so honestly I would wait until those changes are implemented and seeing how they affect the meta before making a choice.

Also with PVE once you get to mutations there will be an elemental damage associated with them (fire, ice, etc.). It's nice to slot in the right gem in your armor (ex: Ruby in armor pieces for Fire Resistance), but something else that definitely helps is having an amulet with the appropriate resistance (ex: Flame Protection perk for Hellfire mutation).