Regardless of its deficiencies in terms of sound quality, the Bruno Walter VPO Das Lied von der Erde with Kathleen Ferrier and Julius Patzak (Decca) is unsurpassed.
Not a symphony, but still one of the greatest of all Mahler recordings: Das Knaben Wunderhorn, Szell, LPO with Fischer Dieskau and Schwarzkopf (EMI)
I've picked up some of the old Tennstedt LPO cycle on vinyl recently but still find his way with Mahler difficult to like.