The 7th has always been a difficult symphony to tie together, Mahler's most problematic. I have always heard it as a set of environments that must be presented with a unifying style, rather than with the inexorable logic and emotion of Symphonies 2, 5, 9.
I’ve dumped some 7s that failed to accomplish this: Kubelik/Bavarian RSO/DGG, Horenstein, and Tennstedt/LPO/EMI and kept others. Klemperer/New Philharmonic/EMI, Solti/Chicago/Decca - neither of these was completely satisfactory – each showing insights, but leaving one unsettled.
Judging by the recommendations here, I assumed the available catalog had changed significantly since I did most of my Mahler collecting, and jumping over to Amazon, I see that is the case. I would have never suspected, nor considered Michael Tilson-Thomas to be a Mahlerian (ducking brickbats). So I listened to some clips on the Internet. It would be ridiculous to judge a complete performance based on short clips, but my impression of the miking is that it is more distant to the orchestra than my recommended recording.
Ricardo Chailly/ Royal Concertgebouw Orchestra/London would be my recommendation to combine performance with sound. Slightly closer miking enables the listener to hear more three dimensionally into the orchestra, especially in the night music passages, picking up details and evoking nature. He seems to bring the sort of unity to the composition described in the first paragraph. Unfortunately it is now only available as a premium price out-of-print find (on Amazon, $32 and $43) or as part of a Mahler Symphony set – which wouldn’t be a bad idea if your library craves one cohesive survey.
Another interesting one is Boulez/Cleveland/DGG, who, I think because of his experience with French impressionist recordings, brings out the mysticism of the night music sections.
I fear we’ll never get the sense of completion from the 7th that we get from the soaring triumphalism of the 2nd or the resolution of anguish and pain that we get from the 9th, but I’ll keep my ears open for even better 7ths.