Honestly, anything that shows off your vocals works in a talent show. Choose the song based on what works best with your vocal range. And stay away from songs over 3 minutes. This girl at our talent show had an AMAZING voice but people got bored after she hit the 4 minute mark on Halo.
I think you should sing something in your vocal range with notes that are easy for you to reach and hit. And also something that you have the lungs to sing based on how soft or intense your voice is. Pick a song that you can understand emotionally. And tempo is important, too; don't pick something too fast or too slow.
Well, I don't know if you're into this kind of music but April Rain by Delain is an amazing song with amazing lyrics too. In case you're not comfortable with symphonic metal (well, this song is not metal really, at all) an acoustic version could work too.
Anyway, other really good bands are Indica and VersaEmerge.
Hope this helps!