hmmm... interesting thread this one.
I can't really pin it down to one single track and I must confess I don't always use the same "testing" material, mostly because I don't test hifi very often probably.
i tend to use a few that i believe will show different abilities. the one thing in common between them is that they are all tracks I know very well.
- This Mortal Coil - "Mr Somewhere" for separation and soundstage (two voices, a guitar and a cello)
- This Mortal Coil - "You and Your Sister" for resolution and detail (the beginning has a whisper that is almost inaudible but with a good system you can even tell what is said)
- Tous Les Matins du Monde OST - "Marche Pour La ceremonie des Turcs" (great track for organic sound - particularly the sound of the drum)
- Beethoven Sonatas "Spring" and "Kreutzer" (for tone and space, balance in presentation)
- Schubert's Fifth Symphony (Beecham) (for resolution and stage)
- Crouching Tiger Hidden Dragon OST - "Night Fight" (amazing drumming with pinpoint placement accuracy)
- Talvin Singh - "Traveller" (to test deep synthesized bass)
- Hossam Ramze "Sabla Tolo" album (any song here uses a mix of tabla and percussion, great for rythm, speed, transients)
And last but not least,
Jacqueline duPre's rendition of the Elgar Cello Concerto. (one for emotion if anything, i know how much this piece has to move me, and i play cello so I can test tonal accuracy, resolution, placement, i just love to hear it basically...

)
My two cents.
Jack