Just one? Sorry, too hard. I'm afraid I'd have to pick from each category that you've said:
Solo: Midnight Train to Georgia, Sarah
Sing-off: As if we never said goodbye, Rachel/Sarah.
Olivers: Give a Little Love
Oliver/Nancy performance: Together, Sarah/Olivers.
If it was just the one - I think I'll go for As if we never said goodbye. It is a musical theatre song, it captures the essence of what this programme was meant to be about, the lyrics are fitting and it's sung by the two best singers & overall contestants in the competition (imo).