The Phantom of the Opera: mainly the Overture transformation. I love it to death. I also love the travelator and candles. However, the Overture will always be the most vivid in my memory of this show.
Evita: I'm a fan of what I call an incomplete set, which is where you do not see a bedroom on the stage, but you see a bed and a dresser and know it's a bedroom. I love how this set moves so fluidly from one scene to another without very many blackouts. The projection screen is another favorite of mine in this show.
Urinetown: Not the Broadway production, but a local community production. I love a set that makes the audience play along with the actors, and this was a great example. The set was one space, bad UC, with two raised areas, one with a skyline and the other a black box and white curtain. Most of the show took place in front of these at the PA, but the UGC scenes happened on the platform with the black box (actually a desk with light-up UGC letters) and the skyline served as a walkway, extension of the office, and the rooftop where the unthinkable happens. For a small community theatre with roughly 100 seats (maybe less), it was really good.
-- SDG