"Best route" is subjective based on your preferrences. We prefer the U.S. highways to the interstate. West of St. Louis; US-50,54,56,&400 would take you West and hook back into I-40. There are palces like Dodge City,
the underground city in Ellinwood,
Dorthy's House in Liberal, KS and
Tucumcari to see along the way.
If you do go to Amarillo,
Palo Duro Canyon is worth a visit.
If you have the time in the Flagstaff area there is
Sunset Crater National Monument,
Wupatki National Monument, and
Walnut Canyon National Monument. Lots lots more not too far south in the Sedona/Camp Verde area.
