Combat Materia curiousity

[Mimilu]

It really depends on the job and what level of gameplay you are doing/planning to do.
-Most jobs prioritize Crit > DHit > Det. This is generally what you can stick to if you aren't doing any higher-end content/trying to min-max/go for world first/etc.
-Skill/Spell Speed is usually for reaching certain thresholds to min-max your rotation. There are a few jobs with secondary Speed builds (like SAM and I think GNB). Like everyone else said, BLMs do love Spell Speed, seeing as they are the most cast-heavy DPS.
-Vitality (and Piety for Healers) can be helpful when you're first learning/trying to clear endgame content for the first time but get flushed out as you learn the fights and get better gear.