It really depends...i wont go into ballista as that is a entirely different ballgame there. General subs for farming are THF or WHM, however theres different ways of doing those as well.

-WAR/MNK is good as a standard tank up until the 60s.
-WAR/NIN is a good standard tank around lvl 51 especially combined with a BRD and they can do decent damage after 40 with a Dual Wield upgrade. Can tank HNMs
-WAR/THF is a good skillchain closer.
-WAR/SAM under certain situations can do good damage especially on lvl 75 parties where you spam WSs and move constantly.
-WAR/RNG is great lvl 30-50ish with Acc up.

-MNK/WAR is the best damage in a exp party
-MNK/WHM vs certain HNMs are great

-THF/WAR is good through all levels
-THF/NIN is great past 24
-THF/MNK before 33.

-WHM/BLM is the standard sub and is great through all levels
-WHM/SMN is a good sub post 50 with Auto Refresh and in HNM fights.
-WHM/RDM is good under 30 and good after 66 for Dispel when you dont have a Dispeller
-WHM/BRD is good after 50 with Ballad when theres no BRD in your party.

-RDM/WHM is accepted as a main healer when theres no WHM or SMN/WHM
-RDM/BLM is an accepted sub for most situations with a WHM or a main healer when you need to focus more on MBs and enfeebling.
-RDM/RNG is a good solo tatic or ballista
-RDM/SMN is...soso not that great, where it has in MP it lacks in abilities and spells
-RDM/BRD is good for fast chaining parties and when you got other mages to pick up where you lack.
-RDM/WAR great for certain BCNM fights, not exp though
-RDM/DRK for Stun in HNM fights

-BLM/WHM is the most recommended sub and most beneficial to the party
-BLM/RDM is good for soloing elementals once Phalanx is acquired and is good for Dispel after 66.
-BLM/SMN is good only for HNM fights but bad for exp parties.
-BLM/NIN is good for Dynamis only.

-PLD/WAR ALWAYS needed in exp party.
-PLD/NIN for certain BCNM fights and ballista

-DRK/WAR under 60 for good damage
-DRK/THF over 60 to avoid death
-DRK/WHM on valkurm dunes to help heal only.

-BST/WHM is a must for solo however they are free to sub whatever they like.

-SAM/WAR is a great damage dealer throughout most levels over time.
-SAM/THF is a great weapon skill closer especially at higher levels and when you load up on STR.
-SAM/RNG is good if you have capped archery and enough ranged accuracy along with good ammo although it might be very hard to hit some things at higher levels.

-RNG/WAR is accepted as a power hitter however they rarely use their power all the way because DEF is paper thin. However its still not a bad sub.
-RNG/NIN is preffered because dual wield is there along with Utsusemi to give tank time to take back hate

-NIN/WAR is a must for tanking
-NIN/RNG is good until the early 40s if you use Guns(expensive).
-NIN/BLM post 51 with Elemental Staves for each spell and will spam alot of powders to do damage(expensive...REALLY expensive).

-DRG/WAR is good throughout all levels.
-DRG/THF is good only as a weapon skill closer even though that is rare to see.

-SMN/WHM is a preffered sub because between bloodpacts healing is the best way to manage that time.
-SMN/RDM is an alternative espeically after 66, WHM is generally preffered though.

-BRD/WHM is preffered for same reason as SMN except that if you are done casting songs magic that is beneficial is helpful