The Crater Lake rim ride is great. It isn't one climb, but it has lots of climbing. You absolutely can't beat the views

The ride up 542 to Mt. Baker is a serious climb and not that much traffic since the road is a dead end. If you really want no traffic there is usually a week when Sunrise road is open to bikes but not cars and often a few days when the North Cascades Highway is also.

Riding on Orcas Island is also not one big climb, but if you include a ride up Mt. Constitution there is a ton of climbing. During the week there are very few cars, even in the summer.