What are, in your opinions, the best (currently available) recordings of the following?
Beethoven - seventh symphony, violin concerto, “Tempest” sonata
Chopin - first piano concerto
Mahler - first, fifth, sixth symphonies, any of the song cycles
Sibelius - violin concerto
For Sibelius, it’s any Oistrakh recording. He brings an incredible breadth to it.
Likewise, I can’t suggest anything other than Kathleen Ferrier for Das Lied von der Erde, itself IMO the greatest thing he wrote. Fifth symphony, I’d say Bernstein. Sixth, maybe Kubelik, maybe Ormandy.
Beethoven, I find it impossible to choose. It depends on my mood 
I’ll second the Oistrakh pick for the Sibelius violin concerto.
An Oistrakh/Ormandy/Philadelphia Orchestra recording of this was one of the first classical LPs I ever owned. A fantastic orchestral-violin duel.