I usually try to mix recordings with few instruments for detail and natural timbre to fairly complex orchestrations to see how much I can hear.
Herbie Hancock, The Piano - For a very well recorded solo piano.
Son House, Original Delta Blues. Especially the tracks where it's just his voice and handclaps.
Maria Schneider Orchestra, The Thompson Fields. Rich, textured orchestration and detail.
Clutch, Blast Tyrant. For good ol' rock.
Pierre Boulez/Cleveland Orchestra, The Rite of Spring.
Imelda May, Mayhem, for female vocals and rockabilly.