I would say it's totally dependant on the material your dabbing, and the quality of flowers.
Are you dabbing BHO that hasn't been properly purged or are you dabbing some full melt bubble, or dry sift.
That bud your smoking, is it organically grown free, of pesticides/fungicides, bugs mold etc, Or is it some BM shwag?
IMO the healthiest thing would be dabbing some pure solventless extracts, followed by vaping some nice clean bud. Rosin has to be up there as well (as long as the bud is top quality and clean)
Edit: Vaping would most likely be the healthiest. Then dabbing pure heads.