The following symphonies are good performances with good sound quality - all on LP...
Symphony 3 - Horenstein/LondonSO, Nonesuch HB 73023 or Unicorn RHS 302/3 (as Newbee suggests)
Symphony 5 - Barbirolli/NewPhilOrch, EMI SLS 785 or Solti/ChicagoSO London CSA 2228
Symphony 6 - Horenstein (again)/StockholmSO, Nonesuch HB 73029 or Solti/ChicagoSO, London CSA 2227
Symphony 8 - Solti/ChicagoSO, Decca SET 534/5 (or London if you can't find the Decca at a reasonable price)
Symphony 9 - Solti/ChicagoSO, Decca SET 360/1 (or London if you can't find the Decca at a reasonable price)
Alternatives for the 1st and 2d you have, both of which will be very different interpretations (not necessarily better, just very different) but in much better sound quality:
Symphony 1 - Horenstein/LondonSO, Unicorn RHS 301 or Guilini/ChicagoSO EMI ASD 2722
Symphony 2 - Solti/LondonPO, London CSA 2217
And, for a change from orchestral, make sure to experience another side of Mahler in his "Five Ruckert Songs." I've always been fond of the Janet Baker performance with Sir John Barbirolli on EMI SLS 785.
Best of luck as you continue to explore Mahler.