Pfew... that really opens a world of possibilities! Anyway, these are my recommendations:
New (or relatively so):
Des Knaben Wunderhorn - Oelze, Volle, Gürzenich Orchester Köln/Stenz (Oehms)
M1 - Pittsburgh SO/Honeck (Exton)
M2 - LPO/Tennstedt (LPO)
M2 - Frankfurt RSO/Paavo Järvi (Virgin)
M4 - Pittsburgh SO/Honeck (Exton)
M4 - Budapest Festival Orchestra/Ivan Fischer (Channel)
M4 - Oelze, Gürzenich Orchester Köln/Stenz (Oehms)
M5 - Gürzenich Orchester Köln/Stenz (Oehms)
M9 - Stockholm SO/Gilbert (BIS)
M9 - Bamberg SO/Nott (Tudor)
Of the Zürich/Zinman (RCA) recordings, you should really check out M1, M3, M7, M9 and M10, but I like the others as well.
I won't comment on 'essential' recordings, as the list is vast and John has already made a very fine start for this.
Good luck, and enjoy!
Freddy