For Mahler, if you strongly prefer instrumental music, I agree with those
who recommend the first and fourth symphonies.  They are relatively short,
as Mahler goes.  I wouldn't touch the fifth at this stage, although its
adagietto is one of Mahler's most beautiful movements.

If you like vocal music, I'd look for his orchestral song cycles,
Songs of a Wayfarer (echoed in the first symphony), Ruckert Lieder and
Kindertotenlieder.  Then Das Lied von der Erde (Song of the Earth.)

Jim Tobin