Streets is the best as it is without a doubt one of the best songs on the album (and of all time) but also encapsulates the sound of the album - the majesty and wide open sonic vistas of the Joshua tree.
Zooropa and Zoo Station also work pretty well.
And I like maglev trains the best, though steam powered trains are also cool.
Streets is definately the best song of all the openers.
But as a "get in in the mood for the rest of the album"-song i'd go with Zoo Station or I Will Follow