I personaly would take dex/qui, since spec buff is better, then base dex. And the increase in cast speed maybe more greater then the extra dmg you would get from more acuity. And since casters get the least hp per con, the little extra HP may not help as much, for helping your grp.

get bases from a base buffer, get DQ from your spec buffer, use an acuity charge or pot or something.

Whatever gives the most dex.

dex > acu > con

I'd always prefer to have/grant buffs that directly address the gameplay and playability, thus making any kinds of Dex buffs the most important you can have on a caster (this may be quickness/haste on a rupt melee). Having a Str/Con buff on a caster is debatable, since he cant use or wont want to use the first attribute, so Conc power is wasted to a certain extend. But I can live with base con on a caster. Acuity should be prefered over base con anyway.

You have to differ between PvP and PvM demands as well. In PvM if you are no (def/aggro) tank and not in direct jeopardy like a bomb you wont need any defense attributes, but it will ask the grp to do their task (e.g the skald not taunting all the time...). If done correctly, only one, maybe two need defense. But this may change regarding to setup/lvling tactics.

In most cases dex is more important than acu

Definitely Dex/Qui, followed by Dex.

If the caster is a damage caster and if there is enough concentration left also give it an Acu buff. Else just use an Acu-Charge.