The following is taken from GameFAQs (Credit DonkeyPunchJr) and gives great details on why certain jobs should be paired with others:
.::God-tier combinations::.
These are the combos that add up to far more than the sum of their parts - so powerful they're practically broken.
Hunter/Breaker: Hunter's ninja swords are one of the best weapons to use with Genji Gloves, and Hunter/Breaker is arguably the best way to do so. This combination gives you 3x swiftness, a decent ranged weapon (hand bomb), all 4 breaks, Genji gear, high strength and HP, and ninja swords.
Hunter/Samurai: Not as strong as Hunter/Breaker but has one huge advantage - this is the only combination that allows for Genji Gloves + Yagyu Darkblade (Dark element weapon) + Black Robes (which boost Dark damage by 50%). In other words, this is THE ultimate Yiazmat killer. You can also easily heal any teammate with dark-absorbing gear by attacking them!
Hunter/White Mage: a nearly invincible healer/tank hybrid. High HP, shields and Main Gauche (a dagger that gives you a whopping 50 evasion) combine with access to all the buff/healing spells to make this the best tank in the game. Even better, you can use the Drain spell to damage enemies while healing yourself.
Black Mage/Monk: perhaps the ultimate offensive magic user. Monk upgrades your Black Mage with 3x swiftness, tons of HP, and access to White Magick 9-13. Being able to exploit Holy weakness is a huge advantage, and the best staff (Staff of the Magi) boosts Holy damage by 50%. This combo also has excellent physical strength, which especially comes in handy earlier in the game before you get all the really good offensive spells.
Black Mage/Red Mage: Another contender for the best offensive mage. Red Mage gets the most powerful elemental spell in the game: Ardor. Unfortunately he gets no fire-boosing gear to make the best of it. This combo lets you use the Flame Staff to boost Ardor by 50%. Oil + Ardor + Flame Staff makes for the highest damage move in the game. A good selection of healing and buffs on top of it.
Samurai/Knight: The syngergy here is incredible. Samurai's katanas do damage based on both strength and magic. Samurai has very high magic power, along with the magic-boosting mystic armor, but has just mediocre strength. Knight has excellent strength and strength-boosting heavy armor, so this combo will greatly increase katana damage. But it gets better. Knight unlocks a bunch of excellent White Magicks, but has lousy magic power. Samurai's magic ability will make those spells far more effective. What's more, this is the only combo in the game that allows White Robes (50% damage boost for Holy element) + Excalibur (Holy elemental sword) + 3x swiftness, making you the ultimate undead slayer.
Samurai/Monk: Similar to the Samurai/Knight combo, this combines Samurai's high magic power with Monk's strength (highest in the game) to greatly boost katana damage. Monk can unlock White Magick 9-13, which benefit from Samurai's excellent magic power.
White Mage/Machinist: This is a nice upgrade for your healer. Machinist gives you lots of HP, 3x swiftness, eventual access to Hastega, and guns (which do fixed damge regardless of your stats, making them ideal for a weak healer-type character). Guns give you a way of doing some decent damage during downtime from healing/buffing. What's more, you can equip Dark Shot + Black Robes to boost their damage by 50% AND heal allies with dark-absorbing gear. Just be sure to set up your gambits so you don't heal any dark-absorbing enemies. If you want to use guns, this is the best combo to do so.
Knight/Time Mage: Time Mage gives the slow Knight 3x swiftness, a ranged weapon (Bowguns), and some decent magic power to make the best of his White Magick. Plus Time Magick, of course. This combo makes for a great attacker with lots of support abilities.
.::Other great combinations::.
Not as absurdly powerful as the above, but still potentially very useful in the context of a party.
Time Mage/Monk: Similar to Knight/Time Mage. Only reason it's not in the "God-tier" category is because you can only unlock his White Magicks once, and Samurai and Black Mage are better contenders for that.
Archer/Red Mage: Archer gets the fire-boosting Burning Bow to boost Red Mage's Ardor spell. You also get 3x swiftness and all the item lores. Overall this isn't as powerful as the Black Mage/Red Mage combo, but it's a good choice if you already have a Black Mage/Monk and you don't want to double up on the Black Mage job.
Uhlan/Time Mage: A great heavy attacker/support character similar to Knight/Time Mage above, but with no healing abilities.
Knight/Archer: Knight benefits from the 3x swiftness and ranged weapon, Archer benefits from the high strength. Not as good as Knight/Time Mage, but decent if you've given Time Mage to someone else and you don't want to double up on jobs.
Uhlan/Archer: for the same reason as Knight/Archer.
White Mage/Time Mage: The ultimate healer/support character. Great if you want to dedicate one character to full-time healing and buffing.
Samurai/Uhlan: katana damage benefits from Uhlan's high strength and heavy armor. Uhlan's Holy Lance benefits from access to holy-boosting White Robes.
Samurai/Breaker: again, katana damage benefits from high strength and heavy armor. Great combination if you want a katana user with all the breaks. You also get hand bombs for hitting flying enemies - a nice benefit for Samurai, whose only ranged attack is Shades of Black (which frankly stinks compared to a proper ranged weapon).
Samurai/Time Mage: upgrade your Samurai with heavy armor, Time Magicks, and a ranged weapon
