I don't feel that I have the qualifications to choose the three "greatest"
works (despite two years of majoring in music in college before I came to
my senses), but perhaps I can choose the three from which I receive the
most listening pleasure.  Perhaps these are unconventional choices (well,
two of them anyway), but if you ask me next week, all three of my
selections will most likely be different:

Haydn: Cello Concerto in C
Beethoven: Symphony No. 7
Vaughan Williams: Symphony No. 5

Honorable mentions: Mozart's Don Giovanni and Clarinet Concerto and Great
Mass, Mahler's Fourth and Fifth Symphonies, Bach's St. John Passion and
Magnificat, Haydn's Creation, Beethoven's Seventh Symphony and Fifth Piano
Concerto, Schumann's Dichterliebe, and many others I can't think of right
now.

John Halbrooks