Before we start, keep in mind you get to pick 3 subjob skills in your 30s, 4 subjob skills in your 40s, and 5 at level 50. So I'll list the top 3 subjob skills with ***, the 4th with ****, and 5th with *****, in my opinion.
PALADINSpeak to Lulutsu at the Uldah Gladiator's Guild, req: 30GLD/15CNJ
Paladin gets several new abilities:
Paladin gets all of the Gladiator skills, mostly weaponskills, defensive maneuvers, and shield skills
And Paladin gets to pick up to 5 MRD and/or CNJ skills, I suggest the following
*** Provoke
*** Raise
*** Cure
**** Stoneskin
***** Bloodbath or Protect
So I suggest getting your Gladiator to 30, Conjurer to 18, and Marauder to 14 to start leveling Paladin.
MONKSpeak to Gagaruna at the Uldah Pugilist's Guild, req: 30PUG/15LNC
Monk gets several new abilities:
Monk gets all of the Pugilist skills, including sneak attack, provoke, second wind which is like chakra, and a bunch of weapon skills
And MNK can pick up to 5 LNC and/or ARC skills, so I suggest the following:
*** Blood for Blood: consumes HP to increase next weaponskill damage
*** Invigorate: regenerates TP
*** Hawk's Eye: increases physical accuracy
**** Quelling Strike (next attack regenerates TP and decreases enmity)
***** Keen Flurry: consumes HP to reduce next weaponskill recast timer
So I suggest getting Pugilist to 30, Lancer ideally to 26 but at least 15, and Archer to 22 in order to level Monk.
WARRIORSpeak to Neale at the Limsa Marauder's Guild, req: 30MRD/15GLD
Warrior gets several new abilities:
Warrior also gets all Marauder skills, including weaponskills, bloodbath (drain), foresight to increase parry rate, and choice of Berserk (increased attack power but lowers defense, cumulative for each successful hit landed, fades when you're hit) or Rampage (increases attack power and speed cumulatively each time you get hit and adds drain effect to crits as long as you are standing still)
Warrior gets to pick up to 5 GLA and/or PUG skills, I suggest the following:
*** Rampart: defense up
*** Flash: like a second provoke
*** Second Wind: cures self HP
**** Sentinel: defense up
***** Blindside (sneak attack, for DDing) or Featherfoot (evasion up, for tanking or soloing)
So I suggest getting Marauder to 30, Gladiator ideally to 22 but at least 15, and Pugilist to 14 in order to start exping Warrior
DRAGOONSpeak to Haurtefert at the Gridania Lancer's Guild, req: 30LNC/15PUG
Dragoon gets several new abilities:
Dragoon also gets all Lancer skills which include weaponskills, invigorate for TP regen, and several enhancing abilities that improve your attacks
Dragoon gets to pick up to 5 PUG and/or ARC skills, and I suggest the following:
*** Second Wind: cures self HP
*** Blindside: sneak attack
*** Hawk’s Eye: increases accuracy
**** Quelling Strike (next attack regenerates TP and decreases enmity)
***** Featherfoot: increases evasion
So I suggest getting Lancer to 30, Pugilist to 15, and Archer to 22 in order to level Dragoon.
BARDSpeak to Georjeaux at the Gridania Archer's Guild, req: 30ARC/15CNJ
Bard gets several new abilities:
Bard also gets all Archer skills which include weaponskills and attack buffs, chameleon for enmity down, decoy to absorb ranged attacks and spells, and a mazurka type skill that increases movement speed to all party members in range
Bard gets to pick up to 5 CNJ and/or THM skills, and I suggest the following:
*** Cure
*** Stoneskin
*** Raise
**** Sacred Prism: makes the next cure or stonekin hit all party members in range
***** Protect
So I suggest getting Archer to 30 and Conjurer/White Mage to 34, in order to level Bard. No real need for Thaumaturge on Bard.
BLACK MAGESpeak to Yayake at the Uldah Thaumaturge's Guild, req: 30THM/15PUG
Black Mage gets several new abilities:
Black Mage gets all Thaumaturge skills which include tons of nukes, sleep, abilities to save or restore MP, and abilities to increase range, power, accuracy and damage of spells.
Black Mage gets to pick up to 5 PUG and/or ARC skills, so I suggest the following:
*** Second Wind: cures self HP
*** Decoy: absorbs a ranged or magic attack
*** Chameleon: reduces enmity
**** Quelling Strike: reduces enmity on next attack (assuming this works on magic attack)
***** Featherfoot: evasion up, restores MP upon evading an attack
So I suggest getting Thaumaturge to 30, Pugilist to 15, and it would be super nice to have Archer/Bard at 42, but at least 2 in order to level Black Mage.
WHITE MAGESpeak to Soileine at the Gridania Conjurer's Guild, req: 30CNJ/15GLD
White Mage gets several new abilities:
White Mage also gets all Conjurer skills which include cures and buffs, a few nukes, some MP restore and enmity reduction abilities, and sleep.
White mage gets to pick up to 5 GLA and/or PUG skills, and I suggest the following:
*** Rampart: defense up
*** Second Wind: cures self HP
*** Featherfoot: evasion up, restores MP upon evading an attack
**** Outmaneuver: increases block rate
***** Shield Bash (if partying and using shield), Flash (grabs target's attention, be careful using it) or Sentinel (defense/enmity up, for solo)
So I suggest getting Conjurer to 30, Gladiator ideally to 34 but at least 15, and Pugilist to 6, in order to level White Mage
In conclusion, while it doesn't make sense at first why some jobs have odd pre-reqs or odd subjobs, after you look at what skillsets they'll be using, they seem to have pretty well-defined roles that make sense. The subjob skills are just my suggestion. You may want to try other skills and see what fits best.