There are several buildings that surround the crosswalk. Is there one or a few that can provide an unobstructed view for photo taking?

I would recommend 5pm onwards, where people start to flock to Shibuya for food/shopping/meet-ups. Any time before, there would be lesser people. The famous place to look at the Shibuya scramble is from Tsutaya's Starbucks second floor. You may wanna head over slightly earlier to grab a window seat, which is very popular, even for locals.

I would recommend Starbucks Tsutaya as well, on the second floor, or the second floor walkway that connects Tokyu plaza to where the Inokashira line is. Pretty good spots!
 

Evening bld near Starbucks 

A rainy day. The photos with all the people and their umbrellas is really iconic!

We've found a metal thing to stand near because it was the only place that was relatively uncrowded. It is very conspicuous so you will probably find it too.