With a drive this long there isn't a 'best'. Mapping programs give you the shortest, or the fastest - based on assumptions about speed on various segments. Generally freeways are given the highest speed, so the route will be dominated by that kind of driving.
Try putting in some intermediate destinations, an see how that affects distance and time. Also think in terms of segments. For example if Oklahoma City is your first major destination, you could just as easily take I70 across Ohio and Indiana, as I40 further south.
I40 from OK to Arizona is a logical choice, since there aren't a lot of alternatives. But even there you can vary the route.
