I actually would say "Me and My Town" and "Simple" from ANYONE CAN WHISTLE over any of the other Sondheim numbers that have been mentioned ('cept for the Tonight Quintet). "Rose's Turn" is definitely one of the most influential showtunes, the way Sondheim experimented with Styne's previous tunes and lyrics from other songs, the way he combined all the themes and motifs, it is simply a brilliant breakdown put to song which followed other numbers like "Live, Laugh, Love" from Sondheim and even a number like "Lot's Wife" from the very recent CAROLINE OR CHANGE is clearly modeled after "Rose's Turn."
I second "Sodomy" from HAIR.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"