The Collaborative International Dictionary of English v.0.48:

Depreciator \De*pre"ci*a`tor\, n. [L.]
   One who depreciates.
   [1913 Webster]

WordNet (r) 3.0 (2006):

depreciator
    n 1: one who disparages or belittles the worth of something
         [syn: detractor, disparager, depreciator, knocker]