Most of the other sections are largely dependent upon what other classes you have leveled as well, since you can't have a different spread for every job.
This above everything else Cid wrote.
Your merits will be ruled by your favorite jobs and your playstyle with them.
For example. Cid mentions INT as a possible 'best benefit'. To which I would say only if your primary focuses are nuking mage jobs. If you say are playing primarily RDM and WHM, I wouldn't go as far Merit INT. This goes further when it comes to RDM considering the fact that a RDM can boost an individual stat based off his at-the-moment needed, and the set shall be complete after the 99 cap raise. (Meaning Gain INT becomes available.)
If you play so much as one melee job habitually you might be better suited off going STR, as you'll probably get better mileage out of it.
The same goes for Combat Skills.
Now I'm all for pushing RDM to its limits, but if you're playing a warrior most the time, Merit G.Axe.
Otherwise, Sword and Dagger are staples for RDM.
Magic Skills (in no particular order) Enfeebling, Enhancing, Elemental are all staples. Prioritize to what you're doing.
But general merits are just that, general. There less about one job and more about what you as a player are doing more as a whole.