Full set of legendary Sentinel armor pieces. Extremely difficult to get all pieces, but grants 15% damage reduction per piece when standing still. I only have 4 on my main character and the damage reduction is better than a maxed set of X-01 power armor (survival difficulty). I've tanked legendary Deathclaws standing still, although they knock you back a few yards. You can even survive the insta-kill turrets quite a bit. They can 1-shot you straight through Mk. VI X-01 power armor with max energy resistance, but it takes 4-6 shots to kill me with just a set of 4 (and their accuracy sucks, fortunately). I've also found some Sentinel melee weapons, which I didn't think was possible since a full set of Sentinel armor would give 90% damage reduction. If it still stacks, that's practical invincibility when standing still.