Leadership - runic harmony should be.
In defense - a battle to the last is mandatory (from my point of view, this is the main skill that dwarves should take).
Helmar, as it seems to me, is not the best option, but I might be wrong.
Simply, Helmar's advantage lies in the fact that he will cast the "Punishing Strike" spell a little earlier (because even without that, after casting "Stone Skin," he won't move too far on the turn order scale).
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