Hello there, I wanted to start a thread like this to help get an idea for some really good combos that exist but perhaps aren't common. Maybe jobs listed here will give you a new spice on an old class? Maybe you'd like to share something yourself? I'm just a regular old fuddyduddy WHM/BLM but that doesn't mean I'm not open to new twists.
One job combo I'll submit to you is the RDM/DRK. A very good friend of mine, Deedlitchan, made me a believer in this and I hope you'll see why as I break it down in early, middle, and end-game.
By Level 10/5
Chainspell
Resist Petrify I
Arcane Circle
All Red Mage spells to 10
By Level 20/10
Chainspell
Resist Petrify I
Fast Cast
Magic Attack Bonus I
Arcane Circle
Physical Attack Up I
All Red Mage spells to 20
+Drain
By Level 30/15
Chainspell
Resist Petrify II
Fast Cast I
Magic Attack Bonus I
Magic Defense Bonus I
Arcane Circle
Physical Attack Up I
Last Resort
All Red Mage spells to 30
+Drain
By Level 40/20
Chainspell
Convert
Resist Petrify II
Fast Cast II
Magic Attack Bonus II
Magic Defense Bonus I
Clear Mind
Arcane Circle
Physical Attack Up I
Last Resort
Weapon Bash
Resist Paralysis I
All Red Mage spells to 40
+Drain
+Aspir
By Level 50/25
Chainspell
Convert
Resist Petrify III
Fast Cast II
Magic Attack Bonus II
Magic Defense Bonus II
Clear Mind
Arcane Circle
Physical Attack Up I
Last Resort
Weapon Bash
Resist Paralysis I
All Red Mage spells to 50
+Drain
+Aspir
By Level 60/30
Chainspell
Convert
Resist Petrify IV
Fast Cast III
Magic Attack Bonus III
Magic Defense Bonus II
Clear Mind II
Arcane Circle
Physical Attack Up II
Last Resort
Weapon Bash
Resist Paralysis I
Soul Eater
All Red Mage spells to 60
+Drain
+Aspir
+Poisonga
By Level 75/37
Chainspell
Convert
Resist Petrify IV
Fast Cast III
Magic Attack Bonus III
Magic Defense Bonus III
Clear Mind III
Arcane Circle
Physical Attack Up II
Last Resort
Weapon Bash
Resist Paralysis I
Soul Eater
All Red Mage spells to 75
+Drain
+Aspir
+Poisonga
+Absorb-MND
+Tractor
+Absorb-CHA
+Absorb-VIT
+Stun
+Absorb-AGI
Now, depending on your race, make any necessary stat mods in strength, HP, MP, weapon use skillcaps, etc. Considering that you can cast "En-" spells on yourself for offense and Phalanx/Stoneskin/Blink for defense (of course our favorite: Refresh) this makes for a very powerful melee Red Mage with plenty of MP to refresh, debuff, and power up for the fight.
Feel free to discuss the pros/cons of this combo here, or post "underrated"/uncommon ones of your own. Thanks! ^___^b