Of the three you already have, PLD will take a lot of work as you pretty much need Ochain and/or Aegis in order to justify your spot in a group like Catwho mentioned. DRK won't be too much of a pain to catch up probably, but is one of the least-sought-after of the main DDs at this time because of its low survivability in an era when DDs are the tanks most of the time. BST is, well, the same as it's always been. Nobody can really use you in a group, but it does well enough in solo.
Of the jobs you're interested in, RDM has limited usefulness anymore. WHM & SMN are my two main jobs and there's no shortage of demand for either one. SMN in particular has made long strides in the last year or so, becoming very valuable in many events. RNG is also extremely high-demand if you have a Lv119 weapon, as it's very useful in some of the really hard content that's out right now like the new Delves, new Ouryu fight, new AA fights, etc. WAR & SAM are two really powerful DDs right now, and along with MNK I would say they are the top demand. NIN is also extremely powerful though not very well-recognized as such, so while there are a lot of uses for it, it won't get a lot of invites unfortunately. GEO has its uses although most people will probably require convincing to let you in, while RUN is still struggling to find its place.
Overall you have a lot of good options in your desired jobs, so I'd encourage you to probably either do DRK first or go straight into leveling up one of your next choices. The reason I don't recommend BST is because most of the newer content, which you'll be missing all the crucial gear from, is group content that you can't really solo very effectively even with modern gear.
Edited, Apr 18th 2014 9:32am by Pergatory