Yours is a question that comes up every once in a while on this list, and
it's a controversial question.  I frankly think that as one of the less
sophisticated listeners here, I'm one of the best authorities on the issue.
OK, maybe that's not such a great argument!  I'm also relatively ignorant
of the choral repertoire, and some of the greats, like J.S.  Bach's St.
Matthew Passion don't generate much passion in me.

First, let you own taste be your guide.  Second, and at the risk of
igniting a firestorm on the list, I would recommend starting with some
of the, um, more approachable music of the romantic era.  You cannot go
wrong with such "warhorses" as Tchaikovsky's violin concerto (respected
even by those who deride Peter Ilyich) or his first piano concerto or
Brahms' symphonies (suggested order:  1,2,4,...3).  Particularly since
you are attracted to choral music, Brahms' German Requiem is one of the
uncontested greats of choral music, and it even receives my seal of
approval.  In general, Brahms was a master of the darker side of human
emotions, and he's readily accessible.  Beethoven's piano concertos are
deservedly among the most frequently performed concertos (start with the
Emperor) but I personally prefer his late piano sonatas- like 28 and 29.
If you enjoy piano solo music, Chopin wrote some of the best:  sample
his nocturnes, polonaises and waltzes and his second piano sonata.

I recommend you not overlook the genre that provides me the greatest
satisfaction:  chamber music.  The Beethoven string quartets are arguably
the apex of the genre (an opinion that some on the list would scorn), but
perhaps Schubert's are more approachable- and his quartet #14 should speak
to your quest for the more sombre side of music (it's nick name is "Death
and the Maiden).  Schubert's Trout Quintet has delighted generations,
although I think I may prefer his Quintet in C Major (Yo Yo Ma has an
excellent recording of this work).  I also would encourage you to get
acquainted with Mozart- you simply cannot go wrong with his violin
concertos (start with #5) or his latter piano concertos- and get
acquainted with the music of Dvorak (Concerto for Cello and Orchestra).

Conspicuously absent from my list of recommendations has been 20th century
music.  So let me offer some suggestions in a feeble attempt to placate the
gods of Musical Sophistication.  Sibelius' second and fifth symphonies are
titans of the century, and are easily approachable.  If you are interested
in opera, try Puccini's Turandot and Berg's Wozzeck, which is (gasp) an
atonal work that is gripping even to someone like myself who is not
generally attracted to atonal music.

You have a wonderful journey on which to embark, and it will likely reward
you many times the effort you put into it.  But it does require some effort
to enjoy this art form, and so its appeal in a world that seeks immediate
gratification will necessarily be limited.  My final advice- and this may
not be easy, depending on where you live- but listen to some live music.
I vividly recall my own experience with Beethoven's string quartets:  I was
rather blase about them until I heard the Emerson String Quartet play his
radical (for its day) Quartet #13.  It was one of my life's more memorable
events, it altered the way I listened to all Beethoven quartets and
completely changed the way I heard that quartet.

P.S.  Samuel Barber wrote Adagio for Strings when he was in his mid 20's.
It remains his best known composition, and most widely appreciated work
(it was performed during Franklin Roosevelt's funeral).  By the end of
his life, Barber resented the work, thinking he would be remembered only
for this youthful, and, I suppose, relatively simple, Adagio.

P.P.S. If you choose to buy recordings on line, try to go through
www.classical.net to support this list and its long suffering moderator!

Larry