Please allow me to get the caveats out of the way before I say why I think this might be the only Das Lied you'll ever need: I'm very much a lay person when comes to appreciating classical music so I can only speak from the perspective of how I experience the music rather than being able to comment on the technical merit of the performance; and, I'm a relative newcomer when it comes to Mahler. I only came across Mahler a few years ago and although I regard his 2nd and 3rd symphonies as the joint-best symphonies I've ever heard, I've barely scratched the surface when it comes to listening to symphonies by any other composer, and I've barely scratched the surface when it comes to exploring the many reading of this particular symphony of songs (I've only heard six or seven).
So why do I think this might be the only Das Lied you'll ever need? The answer is simple: intimacy. This is the only version of this piece of music I've heard that seems to have been played to and for the listener rather than at the listener. Whether that's because of how it was recorded or performed I couldn't say. Perhaps it's a combination of both. All I know is, is that when Janet Baker and Waldemar Kmentt sing, although especially when Janet is singing, I feel as though though they're singing to and for the listener, which in my case, is me. The goes for the orchestra: when they play, I feel as though they're playing to and for me.
There are probably many other recordings out there that sound better, clearer, less polluted by background noise. But I really feel that what is lost in audio quality with it being a live recording is gained in spades in musicality.
Like I say, I don't have the knowledge and experience to critique this performance from a technical point of view. I just want to confirm what others have said: that this is a genuine 5-star version of Das Lied... and probably the only one you'll ever need!
I should add, that although Herreweghe's reading (the Schoenberg-Riehn transcription) comes close in terms of that sense of intimacy, without Janet Baker on vocals, however, it just can't compete with this under-appreciated masterpiece.