This is really tough. Every time I look at them all, I end up not being able to decide between Harriet's staircase, Peacekeeper's ceiling or No_Pro's shell. Can I give each one a 1/3 vote? :-)
Let me think on this some more and I'll narrow it down. Many fine photos besides these three, too. But, these keep catching my eye.
p.s. Peacekeeper - Did we ever find out what your's was a photo of?
edit - peacekeeper - peacemaker - all the same right!