Tenacity

[ColdGuy]

As has been said, make Determination useless for Tanks, make it a dps only materia.

That way, Tanks still have a choice between balancing Tenacity against Crit/DH, but Tenacity becomes meaningful. Wouldn't even need to change it in any way.
Determination is always the last resort stat for high damage builds anyway.

There is no point in doing that, Determination is already a DPS only stat due to the way that gear is made, since Tank gear has no Direct Hit it means that all our extra materia slots are used on Direct Hit, which means that Determination is a no-go stat for Tanks as is. If you really want to push out damage, you go for damage stats which means Crit -> DH, since all Tank gear doesn't have DH, it means all of it can be filled with DH, so nothing ever goes to Determination.

Now, if you were to choose between Determination or Tenacity, then it's even more notorious that Tenacity IS the better option for Tanks already. To test this on paper I made the following sets of crafted gear:

Determination -> Tenacity: https://ffxiv.ariyala.com/1CDZH
Tenacity -> Determination: https://ffxiv.ariyala.com/1CDZJ

With Determination as our main stat we get an added 17.85% damage, and 6.13% added passive mitigation.
With Tenacity as our main stat we get an added 17.32% damage, and 9.39% added passive mitigation.
(These numbers are from the stat weights done a while back on expansion release)


So as far as Tanks are concerned, 3% additional PASSIVE mitigation is a lot more valuable than 0.5% added damage, specially since Tank damage isn't as high as in other expansions, the trade-off is clear and it's in favor of Tenacity.
EDIT: I should add that the total difference, ON PAPER, is 0.1% DPS being traded for 3% Mitigation.

Anyways the difference between stacking one stat or the other is small as hell, who cares, don't let the stigma get you.