For me, it has been Kathleen Ferrier on records, particularly her recordings with Bruno Walter, and in concert & on records, Maureen Forrester. I first heard Forrester sing a Mahler orchestral song cycle in the 1980s, with the Philadelphia Orchestra, and her performance left an indelible impression on me. Mahler is an ideal composer for great contraltos, & Brahms too.
The following 1949 Edinburgh Festival recital sung by Ferrier, with Walter on the piano, shows Ferrier at her best I think:
Here too is a clip of Forrester singing Mahler's Rückert-Lieder, with the RIAS Sinfonieorchester, conducted by Ferenc Fricsay, in 1958: . (It's a beautiful performance, though I still prefer mezzo soprano Dame Janet Baker's recording with Sir John Barbirolli, which is a desert island disc of mine.)
Here also is Forrester singing the Urlicht movement from Mahler's 2nd Symphony, with the New York Philharmonic, conducted by Bruno Walter:
& Mahler's Das Lied von der Erde, with the Chicago Symphony Orchestra, under Fritz Reiner: .