It's hard to pick just one. I think West Side Story, Grease and Chicago are my favorites. "America" is my fav from WSS, "The One I Want" from Grease, and "Cell Block Tango" from Chicago.
Honorable mention goes to "Bali High" from South Pacific.
I actually prefer "Mister Cellophane" from Chicago to "Cell Block Tango". Then again, if I were to ever audition for a local prodcution, I'd be going for the part of Amos, so I might be biased. :)