Many thanks for opening this discussion, ernani. It's a very interesting subject.
To name a few great recording that never were:
Der Junge Lord conducted by Ferenc Fricsay.
The Franck Symphony conducted by Eduard van Beinum (Concertgebouw Orchestra!).
Liszt's
Faust Symphony conducted by Rene Leibowitz.
The complete cycle of Nielsen's Symphonies conducted by Jean Martinon. We only have number 4 now.
The Poem of Ecstasy conducted by Igor Markevitch.
The Rite of Spring conducted by Fritz Reiner.
Walton's First Symphony conducted by George Szell.
Rubbra's Ninth Symphony conducted by John Barbirolli.
I think I'm not so happy with this subject.