I guess we're not all that scientific about it. You can't swing a dead beaver in this state without running into something spectacular. As was suggested, get yourself a Benchmark Map
and just ride.
Northeastern corner, Joseph, the Wallowa Mountains, Hell's Canyon. Must see.
Southeastern corner, Frenchglen, Steens Mountain Loop, the Alvord Desert on the other side of the mountain. Amazing.
Central, Bend, Mt. Bachelor, Cascade Lakes Scenic Loop. Mmmmm.
Northern border, Hood River, the Gorge. Beyond comparison
Coastal. All good, but the farther south the better, IMO
Central/Eastern. John Day, a few different John Day Fossil Beds if you're into that kind of thing
South/Central, Lakeview, Hart Mountain Antelope Refuge, Hart Mountain Hot springs: remote, amazingly quiet, excellent stargazing
Throw a dart and just go.