What makes up the best mashups?

Mashups are practically anywhere in the Internet.  When you visit a website you usually find mashup files via any RSS feeds.  However, not all mashup files that you encounter are what may be considered the best and most effective mashups.  Th best mashups would practically encompass all these three substantial parts:

a. Who provides the data?  For the best mashup to be completed, it requires that a provider of content is identified clearly.  A content provider as the name implies is the source where data being mashed up comes from. The data that you use to mashup is readily available by using any API tool or from the various protocols of the web such as the RSS and REST or the Representational State Transfer. 
b. What is the mashup site?  The mashup site is the site that allows for applications on the web which provides services to the mashup file or site the data from varied resources which are not being owned by the mashup file itself.  This is one of the more difficult things to locate and search.
c. How is the web browser that stands as a client? The client web browser is the actual interface or the working environment that the mashup file or site is actually using.  This interface is capable of being mashed up by any web application particularly the Javascripting language. 

When these three relevant elements are found in the mashup file or site, then you can almost be assured that you have the best and only the best mashup file or site.