I don't have a solid reference set yet, but so far this is what comes first...
Stan Getz - The Girl From Ipanema
Daft Punk - Recognizer
Bag Raiders - So Demanding
Then some Steely Dan, Alan Parsons and David Gilmour.
Female vocal and details: "No need to argue" by The Cranberries
Bass kick: "Take The Power Back" by Rage Against The Machine
Overall bass punch and depth: "Some Chords" by Deadmau5
Detail, speed and Attack: Vivaldi's "Summer"
Dream Academy - Life in a Northern Town
Yaz - Mr. Blue
10,000 Maniacs - Verdi Cries
Mary Chapin Carpenter - Someone Else's Prayer
The Weepies - Please Speak Well of Me
Joni Mitchell - River
Everything But the Girl - Night I Heard Caruso Sing
Killing Me Softly - The Fugees
I used to play technically complex passages when evaluating headphones, but I don't care about that stuff anymore. Too many times I tricked myself into getting excited about a technically excellent headphone that didn't do anything for me emotionally. Now I play:
Opening track from Cornelius - 69/96
Almost anything from Stereolab - Sound Dust
Unknown version of Rimsky-Korsakov - Scheherazade, especially around the 8 minute mark
Sam Cooke - Live at the Harlem Square Club
If a headphone can give me chills/move me on all of these, then it's a winner.
Viva la Gloria (Little Girl) (Green Day) The isolated vocals and the big bassy drum at the start help me find distortion. If it can cope with that drum/bass combo, I'm happy.
Always be my baby (Mariah Carey) Background/foreground sound comparison
Pass Out (Tinie Tempah) Good sound excercise, nice and chip-tuney!
Boys of Summer (DJ Sammy) I know the song very well, so I can hear if something's off.
Get off my cloud and Red Rooster (Rolling Stones) because I need something vintage to offset all the other ones ;)
Pink Floyd - Shine on you Crazy Diamond or the whole of Dark Side of the Moon
Led Zeppelin - Black Dog or Stairway to Heaven
Finley Quaye - Even After All
Paul Simon - Something so right
Nas - Nas is Like
I just try and get a nice variation of different sounds in there
Do Your Eyes See Sparks? by Mice Parade for checking out female vocals.
...And The Great Cold Death Of The Earth by Agalloch, Harper Lewis by Russian Circles and Zodiac by God Is An Astronaut for PRaT.
Dance With Me by Devastations for male vocals.
Welcome, Ghosts by Explosions In The Sky for bass impact (around 1:30).
Nattestid Vid I by Taake for metal.
A Pastoral Theme by Empyrium for acoustic.
Lorelei by Lis Er Stille for indie climaxes.