Which is the best RVW?
ALL OF IT!

Well I am partial to the 8th, which is my favorite piece of music of all time.
As far as his obscure music is concerned,
The Romance for Harmonica, Strings and Piano is excellent, I love his String Partitia, His Piano Concerto, his Oboe Concerto, His Tuba Concerto, all excellent!
There is something about his modal use, his distinct sense of rhythms through the symphonies, his undying passion for folk music, and his sense of classicism that makes his music really stick out to me.
One of my favorite quotes from
Vaughan Williams Studies by Alain Frogley is "the counterbalancing belief in things of the spirit," which is one of the themes presumed to be found in all Vaughan Williams' symphonies. This I believed is really capped in the 8th symphony.
Currently I am reading
Vaughan Williams and the Vision of Albion by Wilfrid Mellers.