What is the best sounding Mahler symphony cycle?

If you like Bernstein, "Mahler: The Complete Symphonies" box set has Lenny and the NYP at their best. It contains performances from the 1960s and has the most amazing remastering of any classical CD I've ever heard.
(DSD-remastered, 2008).

For a single 5th with good sound, try Abbado/Berlin Philharmonic (1993)

Abbado is one of the greatest interpreters of Mahler for sure. I don't know if his most recent recordings with the Lucerne Festival are available on CD, though, they might be just DVD.

Bernstein and Abbado are great Mahler conductors for sure, but the recordings are not really audiophile quality. I still like Abbado best of all on the first, Bernstein on the 2nd, Zinnman on the third, Solti on the 4th, Jansons on the 6th, Bernstein then Abbado on the 6th, Zinman again on the 7th, no favorite on the 8th or 9th, and Zinnman on the 10th. I have not heard any of the MTT recordings.

I can, unabashedly, say that I have heard most of these and find something appealing in almost all of them. However, the OP was asking for "audiophile quality" and, for me, that means a well-balanced and high resolution recording. This eliminates all the Bernsteins, unfortunately.

However, the Abbados on BluRay are superb in every way as are the MTTs and many others on SACD (Chailly, Boulez, Jansons, etc.) Zinman doesn't move me and the sound is only OK, imho.

If you are looking for high quality audiophile approved recordings, MMT's performances are easily some of the best CD's available. As with everything else, however, his performances may or may not be your cup of tea, but I too can recommend his 5th. Zanders performances of Mahler on Telarc are also excellent recordings and include a CD in which Zander discusses the symphony.

Performance wise, if you like your Mahler a little more expressive, consider either Bernstein cycle. You can't go wrong, except with the Vienna Phil set on DG you may want to substitute out the 9th for his recording of it live with the Berlin SO recorded live in 1979, an outstanding performance I think!