Best Route from Jasper to Portland?

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.

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

Orion route is the way to go for scenery.

I agree with Orion that the Rogers Pass on #1 is more scenic than #5. Number 5 is a very easy drive. Should you decide to take it I would suggest a side trip into Wells Gray Provincial Park. It has been far too many years since I have been in there to comment on taking a big rig in, but a day trip out of Clearwater would be interesting. It is different than the National Parks you will have been in. Lakes, Rivers, Waterfalls.
I also agree with what has been said about the Okanagan, pretty area, very busy. If you are into biking or even hiking check out

Myra Canyonsection of the Kettle Valley Railway at Kelowna.
Lots of Orchards and small cottage wineries, particularly south of Kelowna & around the SE corner of the lake near Naramata.

Arguably the best meal we ate on our trip to Alaska last year was a lunch on the veranda of the Quail's Gate Winery in West Kelowna in the OK valley. Table overlooking the vineyards with roses blooming at the end of each row, the big glacial lake at the bottom of the hill, and the mountains in the distance. A summer storm even blew in while we were there and we watched it coming up the lake toward us. Great food, excellent wine, and incredible scenery--what more could you ask? Reminded me of a Canadian version of the Lake Como region of Italy. Don't miss it!

We will be in Glacier, Waterton Lakes,Calgary, Baniff and Lake Louise for about two weeks including two nights in Baniff, and two in Lake Louise before heading on Up the Icefield parkway to Jasper. So on our return trip, I was hoping to find a decent route that has things to see near by and not backtrack to the parkway.
I did see Wells Gray PP but wasn't sure where you get in and can see things, maybe Clearwater? We would probably pick up hwy 97 at Kamloops and take that down to Washington. We have previously been to Grand Coulee Dam and would not go that far back east since we are heading for Portland area (Battle Ground)
Thanks for all the ideas so far. Keep them coming, we still have a couple of weeks before we leave.

Take a look at this Kelowna Website as well. Hard to resist visiting areas that look like this!

Wells Grey is all open now, they plowed the snow off the road last week to get to Clearwater Lake but the rest of the road is open yr. round to Helmcken Falls, still a small ice cone at the falls. Paved rd all the way to the falls and very good shape for all vehicles, dozens of tour busses every day. Entrance to the park is the round about, can't miss it on Hwy 5.

There is always route 5 from Kamloops to Hope on to Abbotsford and cross the border there. You turn off route 1 in Abbotsford just before the traffic starts to thicken up. What the road is like on the US side, well I don't know.

Route 5 is the Coquihalla Highway you may have seen on the Discovery Channel TV show, "Highway Thru Hell" Interesting drive and not really dangerous in the summer.