It's my favorite Broadway score of all time. Picking my favorite part is easy: the Mambo section from the Dance at the Gym. Picking a favorite song is harder, but probably Something's Coming.
Its a tie for me. I think Somewhere is one of the most beautiful and sad songs ever written , and Tonight can still induce goosebumps with the right singer and orchestrations (i.e., lush).
When I directed a high school production of WSS, as my Tony was finishing the song "Maria", a woman in the audience loudly yelled "I'm gonna change my name to MARIA!" and the audience roared with laughter. I, of course, sat there with my arms folded in anger because the audience kept doing things like that.
Edit! I forgot to mention my favorite WSS song. It is the Tonight Quintet. As a child seeing the movie, I thought it was so exciting that each group/person wanted something from the same night. And then to have the realization that if even one of them gets what they want out of the night, then no one else can! It was my painful/pleasurable introduction to dramatic irony and I've been addicted ever since.
Tonight (Quintet) and the 1961 film version of America are my favorites, but Tonight, Something's Coming and One Hand, One Heart are also in there. Hell, the whole score is terrific.