Welcome to the RTA Forum!

"Best" is always a tricky word, because it implies there is one right answer. Any of the trips you've laid out could be done in 2 weeks, it just depends upon what you are after.

Each of your possible plans are pretty different. US-50 would give you a nice broad cross section look at the US from coast to coast. While a Seattle to San Diego would let you focus on all the wonders of the west coast.

I guess it depends upon what you are looking for, but from the sound of your statements, looking for lonely roads and landscapes, I might lean towards the larger variety of a coast to coast trip.