What's the best route from Las Vegas, NV to Vancouver, WA this time of year? I want the least difficult route.?

I want the least mountainous route that will put the least strain on my older, mid-size car. I'm driving that trip about the 5th of April and have found 3 suggested routes on one site alone! I don't know the terrain along any of them! Just want the smoothest trip with reasonable travel time- a little longer is better than winding steep grades!