The essence of Vaughan-Williams can be found on THIS work

[Karl Henning]

I disagree.  Artists are constantly in transitional phases and one could argue they have a "greatest" work that is the summation of their achievement.  Take Shakespeare for example.  He had many facets.  Poetic, romantic (Sonnets and Romeo), vengeful (Titus Andronicus), mythical (Anthony and Cleopatra), comedic (Midsummer Nights), etc., but most consider Hamlet the culmination of all of these aspects so one could argue that is his single work that captured his essence however we understand more of the nuance of the artist the more we read.  That is true with any artist.  Shostakovich's No. 5 captures the drama, dirge, triumph exceptionally well but there are many examples in his oeuvre of any one of these aspects.  Same with Mahler, et al.  There can still be a single work that summarizes who they are and where they'll go.

No, I am afraid that last is an optical illusion.

[Karl Henning]

For many artists their greatest work is the one they are currently working on........

Indeed, one always wants to improve upon the last best thing one has done!