Like everything else, it depends on the context.
Just to mention one perspective, it is common in sociology textbooks to divide religious groups into four categories: church, sect, denomination and cult. According to this definition JWs are classified as a sect rather than a denomination, because they are internally exclusive and externally viewed as deviant, whereas a denomination is inclusive and considered respectable.
The typology is included and discussed in this blog post.
http://benedson.blogs.com/benedson/2005/12/sect_denominati.html
So are JWs a denomination? Not according to one common sociological definition at least.