I was wondering this myself for a while and from experience it seems that...
--SAM seems to be the most useful for just "DD" in about any type of fight.
--DRK looks more versatile for me with its use of more recent spell additions and for zerg (kc/merc/etc).
--WAR is a little harder for me to say but it reminds me of SAM in several ways
--DRG is also versatile like DRK but more so in the sense of what they can do "solo" wise.
The main reason I didn't level SAM myself is because it is limited outside of a party/alliance in what it can do. DRG is very versatile both in and outside while DRK has more choices in a party/alliance (can't say much about outside right now).
For you, I'd say DRG just because, again, versatility and does its job well as DD anyway (great merit job also anyway).
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