+1 relic weakest upgrade for tanks?

[Maelwys]

Does crit have walls you have to meet before it scales to a level that it is desirable? That's my only guess as to why it isn't worth it for PLD but every DPS class seems they can't get enough of it and laugh at DET.

The damage formula is quite complex:
AA = (WAA/WD) x [((0.0032 x STR + 0.4162) x WD) + (0.1 x STR – 0.3529) + ((DTR – 202) x 0.11))] x (1 + (0.5 x (0.0693 x CRT – 18.486)/100))
WS = (Potency/100) x [((0.0032 x STR + 0.4162) x WD) + (0.1 x STR – 0.3529) + ((DTR – 202) x 0.035))] x (1 + (0.5 x (0.0693 x CRT – 18.486)/100))

Basically, it'll depend on what buffs you can stack (e.g. Internal Release!) more than anything else.
Look at the nature of how PLDs build threat... there's a noticeable difference to the weightings of Det on AA damage compared to WS damage, and Tanks basically build threat by spamming WS combos for +Enmity.

Finally, some DDs are very silly and don't know how their own stats weightings work and just assume Crit is always better because of all the nice big red numbers.
(Check this thread and look for the "I think my jump is broken" heading: even for certain DD jobs like DRG, +Det is "better" than +Crit point-for-point!)