well . . . I think on any given day, any professional orchestra in the world could be the greatest Mahler orchestra - for that day, that is. In other words, Mahler has truly become ubiquitous. That fact, more than anything else, has probably forced the Vienna Phil. to take Mahler more seriously in the past few decades. Times change, and people change to some extent. I think they've improved a lot since that Abbado M3 recording from 1980 (or '81). I saw Robertson and St. Louis give an outstanding M5 - as good as any I've ever heard. Mahler is played very well all over the world now. Let's be thankful for that.
Later on:
I see now that you were commenting on the orchestras that Mahler himself had conducted. Yes, point taken. I think they also perform Mahler quite well in Munich and Cologne - two cities where Mahler did conduct his own works. The odd man out seems to be the Dresden Staatskapelle, although Ernst von Schuch did more than his fair share of Mahler in Dresden.
https://en.wikipedia.org/wiki/Ernst_von_Schuch
Mahler is indeed played very well all over the world now and I am thankful for that. This is surely good but it can be also not so good. I mean, it is good because nowadays it is difficult risking to stumble in an awful performance of the Fifth like the one conducted by Maderna in Milan in the early 70s. On the other hand, Mahler risks being played in a run-of-the-mill fashion and this could kill a music that was thought to be thought provoking and even shocking.
(Still, you can have successful interpretations with no particular technical deficiencies like Tchaikovsky's Symphonies with Utah/Abravanel and RCO/Haitink and feel that the Concertgwbouw Orchestra's playing has something more)
That said, in my book, technical deficiencies in orchestral playing has never been a great obstacle in the way of great performances: for example, think about the recordings of M7 and M8 with Kubelik (of course the live ones released by Audite) or about the stunning M5 with the Koelner Rundfunk Sinfonie-Orchester conducted by Rosbaud (ICA Classics, this is an exciting performance that highlights all the modernistic/avanguardistic aspects of this Symphony as very few do - I love it!).
Commitment is another matter. For example, consider the Wiener Philharmoniker's recordings of Strauss music (from Salzburg Festival) with Mitropoulos released by Orfeo and the M9, again with Mitropoulos, released by Andante. It looks like they really care about Strauss while Mahler seems still not to be "one of theirs". Still in the 70s, Bernstein had his problems with the Vienna Phlharmonic, as can be seen in the video with him rehearsing the Fifth (of course he was truly great and in the end he got what he wanted). I really do not like this attitude of the VPO and of Austria in general towards Mahler (and let's face it, it stinks of antisemitism).
Vienna's is not an exception in the Austro-German orchestras conducted by Mahler. The example of the Dresden Staatskapelle, maybe the best German Orchestra, is not an isolated one. As far as I know (if I am wrong, please, let me know), M5 was played by the Berlin Philharmonic, conducted by Horenstein, at the end of the 20s or at the very beginning of the 30s. Then the BPO played it again at the Edimburgh Festival in 1961 (
https://www.pristineclassical.com/products/pasc416 ;
https://www.youtube.com/watch?v=akWajTc1GF8), again under Horenstein's baton. Only when Karajan conducted it in the early 70s, M5 was played again by the BPO in Berlin. They played it 3 times in 40 years, 2 in Berlin and 1 abroad.
Last Autumn I was in Amsterdam for the M7 with RCO/Jansons. I had a chat with a nice Dutch girl seated near me and she proudly stated that that was the 90th time that Mahler's Seventh appeared in a program of the RCO. In 109 years since its Prague premiere, RCO played M7 90 times in Amsterdam. This is the attitude I like and it shows that it is thanks to Mahler's strongholds outside the Austro-German world like Amsterdam (since the beginning of 1900) and New York (after the Second World War) that Mahler's music has been accepted to the point of challenging the popularity of Beethoven's Symphonies.