naysayer

naysayer
    n 1: someone with an aggressively negative attitude
    

40 Moby Thesaurus words for "naysayer":
      anomaly, bitter-ender, brawler, crosspatch, defeatist, defendant,
      denier, diehard, disputant, dissenter, dissentient, dissident,
      freak, individualist, inner-directed person, intransigent,
      irreconcilable, last-ditcher, litigant, misfit, negativist,
      nonconformist, noncooperator, objector, obstructionist,
      obstructive, oddball, opposer, oppositionist, pessimist, plaintiff,
      prophet of doom, protester, quarreler, refuser, resister, sceptic,
      scrapper, sport, wrangler