The blog The Eclectic Ear writes An Appreciation of Joni Mitchell. and they don’t hold back:
[…] in the manner in which we now look back a hundred years or more to the works of Beethoven or Mozart, so it will be with Joni Mitchell. Her music, like that of Duke Ellington’s, will simply stand the test of time.
Read the entire article, which also includes some album recommendations (Hits, Blue, Court and Spark, and Shadows and Light) and some YouTube videos.