I'm posting this in celebration of Mahler's birthday (July 7th, 1860). These preferences change all the time; and there's a lot of Boulez here because the series he recorded over the last decade or so made things so CLEAR. They are great 'study' recordings. With age I seem to prefer the clarity.

Bernstein's recordings are an old preference from discovering Mahler in my teens. And a few years ago I picked up his full cycle with the Vienna Phil that was recorded between his two cycles for Columbia/Sony and DG - I think it's a great cycle, in addition to getting to watch him perform.

And Barbirolli is another old fave - slow tempos that somehow stay exciting.

1st - Bernstein, NYPhil
2nd - Bernstein, NYPhil
3rd - Horenstein, LSO
4th - Bernstein, NYPhil
5th - Boulez, Vienna
6th - Barbirolli, New Philharmonia
(but I also like the Mitropoulos, NYPhil)
7th - Boulez, Cleveland
8th - Boulez, Berlin
Das Lied - Kletzki, Philharmonia
(Fischer-Dieskau, Dickie)
9th - Barbirolli, Berlin
10th - Rattle, Bournemouth

Honorable mentions:

Tennstedt's complete set
Bernstein's complete DVD cycle with Vienna
(recorded between his two CD cycles)
The NY Phil set of historic Mahler recordings

other good recordings of the 6th:
Boulez, Horenstein, Szell