Well... such a vast choice of music to choose from... make sure that you choose something you enjoy practising as you would have to be learning it for a while... What style of music do you enjoy? Classical? How about Beethoven's Bagatelle in G minor op 119 no.1? Neo-classical? ok... Kabalevsky op27 has several great pieces (my long time favourite), or Prokoviev's March or Matin or Tarantelle from Musique d'enfants? Debussy's Le petit negre is delightful, too. Shostakovich's Gavotte from Dances of the dolls is also good example of Neo-classicism.
Jazz? Well, I'm not a jazzer myself and I only play pieces with jazzy feel or rhythm... Mick McCornik, Pam Wedgwood, Christopher Norton... to name a few of my "regulars" in this idiom.
Very adventurous and modern piece? How about a piece from AB's Spectrum? Tsunami from book 1? One of my pupils played this at a recent presentation evening at school, and despite the fact, Headteacher did not like it and felt it was a wrong choice for the occasion, the audience seemed to love it!
Anyway, the important thing is that you don't choose something which is really too hard. All you need is a well performed piece of Grade 5 level to achieve an excellent grade at GCSE.
Good luck!