There are many good symphonies before 29, maybe back to 20? (I'd just get
a complete set.) The Requiem is not really one piece:  there are so many
versions.  You might want to look into this.  I tend to like the (gasp)
traditional ones--I don't like the Levin version at all--but what do I
know:  I even enjoy the Bohm (SLOW).  The Requiem is a minefield.  Maybe
someone so stimulated will write a nice little(?) essay on the subject.
Several have been published.

Operas? Don Giovanni (Giulini) and The Magic Flute (Klemperer--no dialogue,
Bohm). Lots of choices among performances. I like these among the ones I
have.

String Quartets and Quintets? I love Heutling in both, but they're out
of print.  Very dark and warm.  And not to be typecast, I also like the
earlier Juilliard:  much brighter and livelier.  I'm not a fan of the Alban
Berg, Italian, or Amadeus Quartets here.

Also clarinet, oboe, horn quintets.

I have tons of Mozart, but it's all stereo LP stuff, not all in print,
probably, though I'm too lazy to check.  If there is something from this
era you're curious about, let me know, and I'll check.  Otherwise, I'm
sure you'll plenty on the newer stuff from others.

The best violin concertos are probably 4 and 5, for what that's worth.  I
do like the masses, especially the later ones, but others might disagree.

Roger Hecht