It's always the viewer (or hearer, or taster, or. . .?) Artists are just viewers with an inside edge. Despite all attempts to codify and define, art remains the thing that you point at when you say "Art"--at bottom a social activity, not a product.
A critic--they weren't mentioned in your question, but in the past they have at times been granted considerable authority to way what is, or is not, art--is a viewer who is good at communicating a certain perspective on art, one who can persuade others to see artworks in a certain way. If that 'way' adds to someone's appreciation of the work, if it makes their experience richer or deeper in some way, then we may say they are a good critic.
But what they do is not fundamentally different than any other viewer--they must experience, then react with imagination and sensitivity, allowing their world to be changed in the process. It's really that activity in which the art is 'made.'