This was a toughie.
I wrestled with "A Little Bit in Love" also, because I love Audra McDonald's version so much (from the Leonard Bernstein's New York CD). I ended up not choosing it because I can still hear Jennifer Westfeldt's uninspired version in my head, unfortunately. I know it's supposed to be for the song itself ... but, hey, it's my vote!
Here are my five:
Hey There from THE PAJAMA GAME
Look Who's in Love from REDHEAD
Unusual Way from NINE
Moonfall from THE MYSTERY OF EDWIN DROOD
Loving You from PASSION
Itty Bitty Geisha?
Toyland?
Gypsy Pasadoble?
Just a few popular favorites...