You should sing something that truly represents your voice (not simply a sound or a style that you like).
Recommended:
A contest solo may work very well.
A folk song or traditional song may also be entirely appropriate.
A song that you like to sing!
Not recommended:
Songs with mostly one note, and very little change in the melody – this might be popular music, or songs from modern musicals especially. (Operatic recitative is an exception if it is followed by part of an aria.)
Not appropriate:
Heavily stylized pop music, like rock music or belting technique. Our choruses do not perform that style, so that kind of music won’t help the conductor learn about your voice.