In terms of overall flat survivability benefit in full Campaign or higher, the following relics are best in slot:
- DG Proc Heal (best by a wide margin)
- WH Defense
The War Hero defense relic narrowly edges out the DG Proc Absorb by only 0.57%, so either are viable. However, the proc absorb relic has been devaluing *fast* due to the effects of diminishing returns. Defense has a gentler DR curve, and so it will deflate less rapidly than absorb will as we move into ever-higher tiers of gear.
It's also worth noting that some fights actually favor situational mitigation above static mitigation. Toth & Zorn and the hover tanks both demonstrate a lot of tank swapping between a boss that does all m/r attacks and a boss that does all f/t attacks. In cases like that, an activated relic is going to have a much higher average survivability contribution than a static relic, assuming the cooldown cycle aligns with the tank swap cycle.
I actually do have both the activated defense relic and the static defense relic. I use the static defense relic by default (with the proc heal), and I swap in the activated defense relic for certain fights.