Once in Canada, there are two basic routes: eastern route (more inland) and western route (closer to Pacific Ocean). It would be ideal to take one going up north Alaska and the other route coming back down to south.
Up until Canada, the route you take mostly depends on where you start in the lower 48. Someone riding from Florida will have significantly different route compared to another rider from Los Angeles. Both these riders would eventually meet on the same route as they get closer to Alaska.