I get asked this a lot, and all the answers I’ve given come from this source:
Unfortunately he hasn’t worked up numbers for BLM or any of the healers, but all the other jobs are there.
To figure out what materia to meld, look under your job listing on Dervy’s linked page, find the stat with the largest value that your gear isn’t capped in, and meld that until you hit cap (if you do), then move down to the next highest stat. So for Warrior, go for Vitality first, then Strength, then Critical Hit, then Determination, then Spell Speed if you have to.
If you’re not clear on what a “gear cap” is, it’s this: Each piece of gear has a maximum value it can take in every stat which is NOT visibly listed on the gear. If you try to meld past that cap, you will see a red number (usually +0) when you set up the meld, and if you go through with the meld, you will get no bonus and waste the materia. Bummer. The only way to find out if your gear is capped at a particular stat is to set up the meld, although you can also try checking at Ariyala’s site as I think she lists the stat caps on gear in there somewhere. (Just don’t ask me where; I’ve never really gotten the hang of that site. Poke around and see if you can find it, and if you do, leave a comment here explaining what you did!!) Note that endgame gear is almost ALWAYS capped in your job’s primary stat, so don’t plan on melding Vitality to tank gear or Strength to Dragoon or Monk — it will almost certainly not work.
If you meld the wrong materia, you may be in trouble: you can get it removed but you need a crafter class at level 25 (currently) in order to get Mutamix Bubblypots to talk to you and remove materia for you. (He’s at Central Thanalan ( 23.8 , 13.8 ), in the place called “The Bonfire.” Teleport to Black Brush to get to him.) Yes, this is stupid and it sucks. No there isn’t any help for it. Another thing that’s stupid is that if he removes materia for you, he takes out all the materia on the piece at once — so if you melded two materia and only one is wrong, too bad! You lose both! And you don’t get any materia back from this. So be careful with your melds or you could lose a lot of gil.