Tenacity

[Malmstrom]

crit, no mater what, will be always the best because it ( increase critical strike chance , and increasing critical strike damage ) if You ever played Diablo3/PoE where,- crit chance/ crit damage/, as attack speed/ were always crucial to increase DPS, then You would know how stats like this play with each other

two mods at once, thus single mod will be never better

to counter this you would want new materia that has for example, two stats ( inc dmg + as/cs ) at one materia

To be fair, tenacity has a ton of mods.

Affects the amount of physical and magical damage dealt and received, as well as HP restored. The higher the value, the more damage dealt, the more HP restored, and the less damage taken. Only applicable when role is Tank.

Take less damage, do more damage, receive more healing.

Like someone else said, on paper that sounds awesome, but since the values are so low it ends up being not the case.