Best Route from Jasper to Portland?

Orion route is the way to go for scenery.

That is a very scenic route especially this time of year. The fruit trees should have their blossoms. An alternative would be the Icefields Parkway to Lake Loise, west to Sicamous then south.
You could go west from Wenatchee but I would travel further south on 97 to avoid as much of Seattle and I-5 as possible.
Edit: Looks like Orion beat me to it.:D

Many will disagree with me, but I am not a big fan of Hwy 5 from Jasper to Kamloops. It is a good road, but after MT. Robson, there is not a lot to see, with only the occasional peak of a high mountain between a gap in the hills that line this route. Also when you get to Kamloops, you will have to backtrack to get to Hwy 97. Depending on how you got to Banff/Jasper and how much time you spent on the Icefield Highway, I would head back to Lake Louise and take Hwy One back through the Rogers Pass, which is very scenic, and catch Hwy97 at Sicamous to head towards the border at Osoyoos. You'll see how it really dries out as you travel down the Okanagan.
The OK Valley is full of orchards & vineyards with lots of tours. It has several nice towns and although busy, is quite scenic as you drive along the 80mile long Lake Okanagan and several others, including Kalamalka, which means 'Lake of Many Colours'. If the light is right, it is very pretty. Lots of campgrounds, but very busy in the summer.
After the border, you could stay on Hwy 97 and take a couple of diversions to Lake Chelan, great campground there, and to Leavenworth, which is a very pretty Bavarian town.
The alternative is to take the 155 at Omak and head over to The Grand Coulee Dam. This is quite the sight and when you stay on this road you go right by Dry Falls, which is stunning, as long as you realise what you are looking at! This whole are is very scenic with dramatic rocks & lakes.