I have never been so happy for a necropost in my life, I don't think.

I didn't know this thread was a thing. Apparently the 35 point : 1% Parry conversion is as of Heavensward. Feel free to help me shove my foot into my mouth on that particular data point.

That said, let me math some. It takes 350 Parry to mitigate 2% of physical damage (350pt*1%/35pt = 10% *20% = 2%), when we compare that to Crit gains, according to the coefficient a few posts above you're trading right around 8.16% Crit for that 2% mitigation. When we adjust total damage dealt, assuming flat base stats, it becomes 91.84 + 150%*8.16 = 104.08 = 4.08% damage boost. Now, this doesn't account for the exponential scaling provided by Crit, but regardless, let's work with that 4.08% figure a minute. Let's give tanks the benefit of the doubt, and say everyone contributes even damage for the duration of a fight. In a 4-man group, that translates to a 1.02% increase in group DPS. It is doubtful that killing it 1.02% faster will outweigh 2% physical mitigation provided by equivalent Parry. So let's ballpark some more coefficients here: we'll say only 70% of incoming damage is parry-able, and you're not being attacked for 20% of the fight, so 80%*70% = 56%. And let's also say that tanks can contribute only 15% of a small party's DPS (so 60% of what we had before). That brings effective parry mitigation to 1.12%, and group DPS boost from Crit to 0.72%. Unless killing the boss .72% faster will mitigate more damage in a small party than the 1.12% mitigation you get from Parry, Parry is still the best option for mitigation.

Now, thanks to the exponential nature of Crit scaling, there IS a threshold where Crit will cause a tank to do substantial enough additional DPS to mitigate more total damage than equivalent Parry. But I have a sneaking suspicion that they built it in such a way that top tier gear pre-4.0 will cause them to be equal. It will also take a calculus equation to determine the actual effectiveness of Crit, which I am too rusty at math to perform (it's...been a decade since AP Calc. I feel old. :|). It is also difficult to gauge the additional value of Crit for WAR and DRK, who employ self-heals based on damage dealt, without comparing amount healed to amount damage received - data which is inaccessible without a ToS violating parser (or hours of meticulous analysis).

It's also worth noting that fights with extended periods of no damage and/or high magic damage will degrade Parry's value, and similarly, fights with extended periods of time wherein the boss cannot be attacked will degrade the value of Crit. Parry also has very little value for someone in an off-tank spot. And it is further of note that Crit's effectiveness is cut by at least half in a Full Party, while Parry's effectiveness is untouched.

So for main tanks of either Small Parties or Full Parties it seems that Parry has much more value for mitigation than other stats. However, for off-tanks, it seems like it's still advisable to optimize DPS.