My choice would be a song cut from the original ANNIE GET YOUR GUN and never heard again until it turned up on one of the LOST IN BOSTON CD's - "Take it In Your Stride." A Good old fashioned snappy show tune by Irving Berlin.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com