Agree re Dvorak's 1 to 5 - as well as the Kubelik and Kertesz sets I also have cycles by Rowicki, Suitner and Gunzenhauser. All are fine works but IMO the Rowicki 1-3 are even better than the Kubelik and Kertesz.
No mention yet of Bizet's Symphony in C - a big fave of mine, having the classic Beecham recording and equally excellent ones by Marriner and Abbado
Rimsky -K's 'Antar' symphony - Zinman and Svetlanov versions both excellent
Tchaikovsky's first 3, well overlooked IMO. Just check out the Janssons, Karajan and Rostropovich sets to see why.
Borodin's 2 and a half - I love pretty much everything I've heard by Borodin. Shame he didnt do more. Amazing to think he was a hobbyist composer and a top flight scientist
Franz Schmidt - his 4 symphonies are terrific -why so overlooked?