This subject has been discussed previously however, given the amount of hi-res recordings now available it is worthy of an update!
Reference tracks are really useful sources of information; for A/B comparisons to check balance, positioning, distance, reverb level and EQ. Large scale orchestral repertoire containing soft and loud dynamics, a variety of orchestration, solo/section phrases and full tutti are obviously the best source for this. Of course it is very subjective and down to personnel taste, however here is my selection of great performances/recordings, from purist/clean recordings to the most bombastic film recordings; all are available in hi-res formats (24 bit 48 kHz upwards). All excerpts adhere to the 10%/30 second Creative Commons fair use guidance for educational purposes.
SACD, HDCD and DVDA disks are more readily available at the moment from companies like Reference Recordings, Denon, Chandos, Pentatone, Telarc and Linn to name but a few. There are also Hi-Res downloads readily available from sites like HD Tracks, Presto Classical, Channel Classics and Native DSD Music; the latter having free downloads of Mahler 1 in 32 bit 352kHz (uncompressed, unedited, unprocessed) format!