I notice that LA is still unrepresented in this discussion.
For me, the most beautiful building in the LA area is the Wayfarers Chapel in Palos Verdes by Lloyd Wright (Frank's Son).
others might prefer Gehry's Disney Hall
or IM Pei's UCB building
or Richard Meir's Getty center
or Pierre Koenig's case study house #22
or Phillip Johnson's crystal cathedral
and then there's........