Which DPS is the Tankiest?

[Silhart]

Individual stacks did not increase the mitigation however, it was consistent at 10%. It simply expired after three weaponskills.

Riddle of Earth was indeed 30% mitigation at full stacks. Here's a screenshot from The Balance server taken on the day of patch 5.5's release:

The numbers here are consistent with what you'd expect from a 30% reduction in damage