Dark Knight will almost certainly come from MRD or GLA. The iconic weapons setup for Dark Knight is one-handed sword and shield -- almost every case has this setup (sans optional axes in WotL or the Black Knight of CC, then the lack of shield in X-2, then FF11's scythe or alternate greatsword). Additionally, a Dark Knight has the mechanics needed to beat the tank out of GLA, meaning it's actually a viable DPS job for that truck of a class. However, in terms of actual skillset, MRD is perfect for a Dark Knight. You couldn't make a better set of abilities for a Dark Knight if you started from scratch. Take your pick, but there's really no good reason to make a Dark Knight out of anything else.