Me. I don't care what the horse looks like standing: I want to see how he moves.
I have seen a couple of old (former) champion horses that looked like nothing when standing in the field, in fact one was a broken-down wreck. But when a rider got on either one, it was a magical transformation. Both those horses became poetry in motion.
That's what I look for: how the horse moves. The other thing is a nice disposition.