Could use help in picking Drk, Gnb, or Pld.

[Seku]

DRK is going to be better at tanking Tank busters in most fights under Savage fights (since I've not done those). TBN, in my opinion, is the best synergy defensive ability of any tank at the moment due to the fact that since it's shielding it doesn't lower any additional cool down.

As for your main question, pick which ever you want to play and feels good to you.

GNB seems to be the most active. Very good burst damage.
PLD can save a run with spam cures. Though I think it's new AoE makes it pretty strong for trash pulls.
DRK TBN is pretty strong, but requires that you have a good notion of when it's going to break or not or you basically lose a Edge or flood of shadow.

[Hierro]

When you take any sort of tank buster, you'll want to use your on-demand (TBN, HoS, sheltron, RI/NF) with either rampart or your 30% redux CD. Using only your on-demand is cutting it too close for anyone's comfort. Then again, that's only the sort of mitigation you'd want for more hardcore content.

In my opinion, GNB has a bit of the skill ceiling because it's the sort of job you want to pre-cast defensive cooldowns to avoid double weaves during potential GF or NM windows. DRK is definitely more beginner friendly due to it's relatively low APM and TBN can be paired with anything and give great results.