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.
I think the Mambo is some of the most exciting dance music ever written for Broadway. Something I really love about Bernstein's music is his ability to stretch it to nearly the breaking point, both melodically and rhythmically, and the Mambo is a terrific example. It comes so close to falling apart; the melody is bizarre and full of weird dissonances, and the syncopation is so extreme that it feels one beat away from being nonsensical. Both elements, though, are just held together enough to create this thrilling rubberband effect, like riding the Tilt-a-Whirl, where you're constantly being whipped back into the centre and then flung back out. It's just phenomenal, and it's also thematically relevant! The music is just like the rival gangs, meeting up and dancing on the edge of catastrophe.
Would anyone judge if I said “Somethings Coming”? In my social media feed a clip popped up of a cover of the song by some in Glee and I just....fell in love.