Registered users can then post on the site their Dream Mahler list of the "ultimate" recordings of Mahler symphonies. It's a good idea, which can be later applied to Beethoven, Bruckner, etc. and might serve to open some of the buried riches of the catalogue to the casual (or expert) listener). Unfortunately, Das Klagende Lied and Das Lied von der Erde have been left out of the "Dream."
Anyway, here's my list. Please keep in mind that this only includes recordings that are available from Universal Music Group. It should really include something by Simon Rattle or Klaus Tennstedt, the new Bernard Haitink Mahler recordings with the Chicago Symphony Orchestra, the first round of Bernstein/New York Philharmonic recordings or anything conducted by Michael Tilson Thomas. C'est la vie.
Symphony No. 1 "Titan": Royal Concertgebouw Orchestra cond. Leonard Bernstein. (1989)
Symphony No. 2 "Resurrection": New York Philharmonic cond. Leonard Bernstein (1988)
Symphony No. 3: Berlin Philharmonic cond. Claudio Abbado (1999)
Symphony No. 4: Philharmonia Orchestra cond. Giuseppe Sinopoli (1993)
Symphony No. 5: Vienna Philharmonic cond. Leonard Bernstein (1988)
Symphony No. 6: "Tragic" Vienna Philharmonic cond. Pierre Boulez (1995)
Symphony No. 7: "Song of the Night": Chicago Symphony Orchestra cond. Claudio Abbado (1984)
Symphony No. 8: "Symphony of 1,000": Chicago Symphony Orchestra cond. Sir Georg Solti (1972)
Symphony No. 9: Royal Concertgebouw Orchestra cond. Bernard Haitink 1970
Symphony No. 10 (first movement only): Vienna Philharmonic cond. Leonard Bernstein (1991)
Because I'm a completist type, I would add the following recommendations:
Des Knaben Wunderhorn: Royal Concertgebouw Orchestra cond. Leonard Bernstein
Das Klagende Lied: Philharmonia Orchestra cond. Giuseppe Sinopoli
Das Lied von der Erde: Vienna Philharmonic cond. Pierre Boulez
Of course if all this is too much, you can just check out the new Mahler: Complete Edition boxed set, which compiles a "best of" from the DG and Decca catalogue and represents some of the finest available recordings. Some of them are even on my list!