Brandywine River Museum, Philadelphia
This is the place where three generations of the Wyeth family created their art work, and the collection for their work here is unsurpassed anywhere else. You can see their art work, you can visit their very studio, or you can just get a sense of the area that inspired them.
There are collection from other artists, and changing galleries on interesting topics. But, if you're not into any of the Wyeth paintings, this place isn't worth the effort.
But if you ARE, then you can't miss this place.