/SAM= Bread and butter, gives us the most tools to get the job done and happens to be the best DD option in 95% of cases(except in VW). Use this always unless indicated otherwise.
Pros:
+Hasso, sTP for TP gain and haste
+Sekkanoki for self skillchain or TP overflow(Drakesbane->Stardiver= Gravitation)
+Seigan&Third Eye will keep you alive in situations where you have hate i.e temporarily tanking, soloing old content like Limbus, too much hate etc)
+Demon Killer/Warding Circle help for demons which aren't that uncommon these days i.e Ig Alima, Dynamis-Xarc mobs.
Cons:
-A big con used to be healing but now with Restoring Breath this is mitigated to a great degree.
-Can't remove debuffs/ailments
-Attack is lower than with /WAR, although the pros really do outweigh this, especially the more magic haste you have.
-Lack of provoke may be a problem in low man situations where the tank is dead and you need to grab hate off of mages.
/WAR= Mainly for voidwatch or epeening damage in Abyssea.
Pros:
+Massive attack bonus from Attack at the cost of defense. In situations like VW where TP gain is more related to regain/Dusty Wings and not Store TP or attack speed, this plays a more vital role in overall damage than anything /SAM will net you.
+ 10% double Attack is very nice, can proc on Jumps and Weapon skills for curiously high damage/TP gain on a proc, can give this trait to the wyvern with Wyrm mail.
+Provoke actually is very handy when low manning content with a lot of 'lightly armored' jobs or mages and the main tank dies. Since our jumps either make 0 hate or shed hate, this can actually help you take control of the situation while everyone gets back on their feet.
+Defender isn't worthless, it can come in handy in the situation above, though not as effective as Seigan+Third Eye but maybe after the DEFENSE update, it will have more use, especially in conjunction with a PDT set.
Cons:
-While Berserk is nice, it can lead to you dying very fast. When berserk wears, most of the benefit of /WAR is lost while you're waiting for the recast.
-/WARs other job abilities like warcry are lackluster and don't really benefit you or your party by much.
-Lack of sTP traits means it's harder to get a 5-hit build which greatly impacts TP gain and the amount of swings necessary to reach 100%+.
/RDM
This is really the go-to subjob for soloing for many, many reasons. The spell list is huge and /RDM has many tools for a variety of situations and is my personal favorite.
Pros:
+Haste, a 3 minute 15% haste spell is extremely useful.
+Dia/Dia II, while dia makes a decent pull spell or HB trigger spell, Dia II is amazing for soloing semi hard or difficult fights or for chaining a lot of exp in quick succession. 10% defense down is nothing to scoff at especially when combined with Angon for a total of 35% defense down. To give you an example of how amazing that is, if you assume you have 1000ATK and the mob has 1000DEF, 35% defense down is about equivalent to you having +50% attack.
+Enspells, while seemingly weak, 9-12 damage every hit over 3 minutes can add up, especially on mobs that take extra damage to magic or have invincibility or something where they are immune to physical damage. Curiously enough the damage from an enspell is added on to your jumps and weapon skills although not indicated in the log, the damage is added already to the final damage total. Empathy this to your wyvern for extra benefit.
+Ice Spikes: I don't know if it's intentional but from my anecdotal experience, Ice Spikes' paralyze sticks about 15-25% ignoring level difference. Meaning the mob will receive the effect of the paralyze about 1/4 or 1/5 hits. As for the proc rate of the actual 'paralysis', it's about as potent as Paralyze II I'd wager. When I solo DC mobs in Dynamis Qufim as DRG/RDM, Ice Spikes paralyze has saved my life and procs extremely often, about 20-25%, I've seen it proc 4x in a row before, it's amazing.
+Cure IV, enough said
+Shell II, ProtIII, enough said
+Barspells: Amazing HB triggers and decent vs magic
+Sneak/Invis: enough said
+Dispel: Useful!
+Refresh: Godsend if you don't have autorefresh gear or ethereal earring yet!
+Convert: See Refresh
+FAST CAST: Amazingly useful, helps more than you think for HB triggers
+Magic Defense Bonus: Again helpful more than you realize
+Stoneskin/Blink: good as prebuffs before a difficult fight
+Aquaveil: prevent pesky interruptions to your HB trigger. Godsend on a HF mnk mob or fighting multiple mobs
Cons:
-Hard to remove debuffs, have to rely on Wyvern remove breaths
-Lower defense against very physical strong targets(like Orcus) in cases like that, /BLU shines a great deal more than /RDM.
-Lack an OMGWTF Divine Seal+Cure IV
-Lack of Banish could hurt you against skellies
/WHM
Pros:
+Can remove debuffs
+Banish: Useful vs skeletons
+Emergency Cure V with CureIV+DS, good if Wyvern dies and you need a little extra healing.
+Most of the same benefits as RDM, not gonna type em out
-Cons
-Everything black magic or trait related that /WHM lacks from /RDM is a con. Really /WHM is inferior and almost never used over /RDM or /BLU.
/BLM
--Don't bother with this, the only useful thing about this is Stun which is overrated if you're soloing unless it's for a specific mob/fight where you absolutely need it for some reason.
/BLU
-Practically as useful as /RDM, slightly different
Pros:
+Cocoon is mainly why this is used, +50% defense for 90 seconds means you can get crazy damage redux for certain hard fights
+Sprout Smack is a great HB trigger spell, a bit more MP than Power Attack or Rabbit Kick but the chance of sticking the added Slow Effect is quite high and the slow effect is moderately potent ~10-15%.
+Sandspin, this is similar to Dia in that the Accuracy Down almost always lands which could help if you're fighting something hard and need the extra debuff.
+Metallic Body: Can give you a decent 100ish Stoneskin, good prefight.
+Headbutt is another good HB trigger though more expensive, I alternate Headbutt with Sprout Smack, stun is moderately accurate.
+Refueling: Not as good as haste but better than nothing.
+Can set spells for traits i.e Killer or Auto regen, helps decently; also setting spells modifies your base stats, can get a decent boost to HP, VIT and STR by setting certain spells.
+Due to how BLU spell damage calc works, if you're fighting TW mobs, Bludgeon or Jet Stream actually deal quite high damage (400-600)
+Can dispel
Cons
-Lacks sneak/invis
-Lacks a defense down dia
-Unless you have a way to recover MP with gear or items, can run out of MP
-Wild carrot isnt as good as cure IV
-Poor against magic, no MDB or shell or barspells
-Must learn spells
-Some spells will 'fail to activate' if they fail an accuracy check.
-Less utility spells