Debussy - Syrinx
It might take less than three minutes to play, but Debussy's Syrinx crams a lot into one piece for solo flute.
Any flautists out there, no matter their standard, will no doubt have a soft spot for this intricate little gem from Claude Debussy. His impressionist sound is more commonly seen on a large orchestral scale, but with this dainty miniature for solo flute he proved that the flute alone is more than capable of encapsulating the impressionist movement.
Budding flautists all end up playing this work at some point, and it's easy to see why. There's a lot of room for interpretation, it sounds impressive and, most importantly, it's been a hit with audiences for a century. It had slightly more inauspicious beginnings, though - Debussy initially write it as incidental music to a play that was never completed.
If you want to get really technical about it, it was also the first solo piece composed for the modern flute (using the Böhm system of keys and construction). Even more strangely, Syrinx is often performed offstage, as something of a musical aperitif in concert intervals.