If you had not included "the absolute most" at the end, I would have answered "everything". Truth be told, everything can inspire me, and sometimes epiphanies come to me in the most bizarre ways.

But since you're asking me what inspires me THE MOST, I'd have to say there are two things. The first, is ambiance. What I'm referring to specifically, is getting a distinct "feeling" out of scenic pictures or places. Here, let me show you three pictures that I really like:
http://i55.tinypic.com/mcdv5w.jpg
http://i53.tinypic.com/282q7a0.jpg
http://i55.tinypic.com/10cjcp5.jpg

And also that of the industrial or derelict variety:
http://i53.tinypic.com/167tfo5.jpg
http://i56.tinypic.com/mm7578.jpg
http://i51.tinypic.com/2ibdz5t.jpg

The second thing that really inspires me is modern architecture. Technically, modern architecture does go hand in hand with ambiance.
http://i53.tinypic.com/2hhiz5e.jpg
http://i51.tinypic.com/atnofd.jpg
http://i54.tinypic.com/mw2mpv.jpg
http://i56.tinypic.com/9zpt3s.jpg

Those are just a few of my examples.
Both of these things evoke strong feelings inside me, and thus, inspiration is formed.