Cliff,
On its own, I think the U.S. Open's stature has been enhanced in recent times. It's no longer the predictable slog through high rough – Mike Davis' set-up and the excursions to new places like Erin Hills has seen to that – and even under-par scoring seems more welcome.
However, when he was USGA president, Glen Nager saw it differently, and that was one reason Fox is now the television rightsholder rather than NBC. Nager saw the Masters surpassing the U.S. Open, at least as a television spectacle. For that, credit Augusta's iron hand on the throat of CBS. The Masters is elegance in golf, presents itself as that, and succeeds. The National Open is everyman's championship, if you qualify.
I see the two, like a blonde and a brunette, different but similarly alluring. It's all to one's taste.
The British Open, as Henry Longhurst called it, is the siren from across the sea.
The PGA? Great championship, and moving to May should eliminate the possibility of fainting on a hot course. Given it doesn't include amateurs, I think its just a step below the aforementioned three, but still major (as are the U.S. and British amateurs).
Good topic!