naysayer

noun

1. One who consistently denies, criticizes, or doubts; a detractor.

   “He ignored the naysayers and succeeded with his idea.”

   “As for the IRP, Secretary of State Grant Shapps continues to peddle snake oil, smoke and mirrors. His reaction to near-universal IRP condemnation from politicians, local and national media, and all but a few rail specialists was to dismiss the lot of us (in the condescending and patronising tone we have now come to expect) as "critics and naysayers".”