If you want a scenic drive and avoid stop and go traffic, go White Pass, but plan for a scenic drive with winding roads, a pretty good incline and good hill going down the pass. This is my choice if I don't want to sit in stop and go. This is also our newest destination for camping - but before you go, check the fires in Yakima. Not sure if any fires are effecting the roads over there.
If you want the easiest drive on your vehicle, lots of freeway, truck traffic, stop and go, then go 90. Once you get to North Bend (or off 18) it is pretty good, freeway travel. This is also your fastest option, depending on how much rush hour traffic, or just plain overcrowded.
If you haven't done White Pass, I would recommend that for the view and scenery.
Don't go Portland to 84, unless you are trying to avoid a pass. Long boring and lots of traffic.