Ralph Moore has written so much that there is very little to add, although I do not agree with everything he has said. It is fascinating to hear the "Tennstedt sound", which might almost be called the Mahler sound, brought to other great composers. His ability to make every voice in the orchestra clearly audible without any one of them dominating, is for me his sonic trademark.
The pick of the bunch is Mahler 1 with the Chicago Symphony Orchestra, so much more involving than his rather under-stated performance with the LPO; this also has the best recording quality. The two Bruckner symphonies come next, especially the Eighth, in which he brings out a lyricism and humanity which not all conductors find. The Brahms German Requiem is compelling despite very slow tempi in the first and last movements. The baritone Jorma Hynninen is very good, however Jessye Norman, a singer whom I admire, is not at her best here.
The Beethoven Third, Sixth and Eighth are first-class even if they do not erase memories of Klemperer in the Eroica and Boehm in the Pastoral. In fact there is very little that is not recommendable; I have to say I was not impressed by the "New World", which seems to lack its Czech quality, but maybe that is just me.