Debussy- Syrinx
Bach- Sonata (Partita) in A minor
Telemann- Fantasies

Flute World also has an extensive selection of music for unaccompanied flute.

Some others:

Honneger: Danse De La Chevre.

Hoover: Kokopeli or Winter Spirits. (She has several solos. But I'd really recommend Kokopeli.
http://www.youtube.com/watch?v=xsppVeCD2WQ

C.P.E. Bach: Sonata in a minor

Jacques Ibert Piece:
http://www.youtube.com/watch?v=cTioPjBEm4c
or
http://www.youtube.com/watch?v=YlJNICd_WE4

Bozza: Image

http://www.youtube.com/watch?v=alzS8hPsOfI

There are tons of solo things including etudes and studies which are impressive -if they are played musically.

flight of the bumblebee- i love that song, other then that i don't know anything else thats differicult