Robert Peters strats of his life on this list with an impossible question:

>So this is my question: Which, say, 25 works, do you think are absolutely
>essential for every serious classical CD collection? The works should be
>of every period (from the Middle Ages to the present) and from every genre
>(opera, Lied, symphony, chamber music, concert, etc.).

25 works from the history of music is triflingly littlish, hoping that
there will be enough answers, I'll give 25 from my area of competence: the
20'th century.  in no special order.  I'm not sure if Robert want specific
recording suggestions but I've include them anyway..  availability? Who
knows?)

1: Bartok - string quartets, start with No. 4 (Vegh QT on Valois)
2: Shostakovich - string quartets, start with No. 8 (Yggdrasil QT on BIS)
3: Stravinsky - Rite of spring (Markevich on EMI/TESTAMENT)
4: Bartok - Piano music, start with "childrens album" (Kocsis on HUNGAROTON)
5: Messiaen - Turangalila symphony (Chailly on DECCA)
6: Schoenberg - Pierrot Lunaire (Eotvos on RCA)
7: Xenakis - Pleiades (Kroumata on BIS)
8: Ustvolskaya - Composition No. 2 (de Leeuw on PHILIPS)
9: Ligeti - Le grand macabre (Salonen on SONY)
10: Webern - Songs (Oelse on DGG)
11: Nielsen - Symphony No. 4 (Blomstedt on DECCA)
12: Rosenberg - Piano Concerto No. 2 (Widlund on Daphne)
13: Crumb - Ancient Voices of Children (De gaetani on NONSUCH)
14: Cage - Sonatas and interludes for prepared piano (Berman on NAXOS)
15: Prokofiev - Piano Sonata No. 7 (Pletnev on DGG)
16: Shostakovich - Symphony No. 13 "Babi Yar" (Kondrashin on RUSSIAN DISC)
17: Weill - "Mahagonny" (Latham-Konig on CAPRICCIO)
18: Eisler - Songs (Busch on BERLIN)
19: Gubajdulina - Und: das fest ist in vollem gang (Geringas on COL LEGNO)
20: Tan Dun - Ghost Opera (Kronos QT on NONSUCH)
21: Norgard - Symphony No. 3 (Veto on DaCapo)
22: Takemitsu - Quotation of a dream (Knussen on DGG)
23: Berio - Visage (Berberian on BMG/Ricordi)
24: Schaeffer/Henry - Symphonie pour un homme seul (Composers on INA)
25: Andriessen - De Materie (de Leeuw on NONSUCH)

A very biased list, and 25 is really far to small a number, this does not
even tuch the top of the surface...  No Ives, No Mahler, No Skalkottas,
No Berg, No....  Yes I've consciously left out the "a Capella choir" stuff
couse thats one thing I'm really not in to.  Othewise I think that I have
most bases covered, even if I would have liked to have include more
electro-acoustic music and something rally contemporary (written this year,
but I dont know anything has surfaced on CD yet..)

peter lundin, gothenburg.se -  Counting the days: DSCH 100 (1906-2006)