Kind of depends
With all the guys in the Portland and Battleground areas, I'm sure you'll get a ton of recommendations, but I suspect they all start with The Gorge!
Coming from the Puget Sound area, we normally converge on a pretty common routing. I've gone the southern route through Twin Falls, ID, Scottsbluff, NE, then into southwest MN, eastern SD, or eastern NE (I like O'Neil) for the night, getting you into OSH after a short hop the next morning. Or, there's the more direct route through Helena, MT, and out over MT and ND.
Due to weather considerations, you might plan out a couple options as large Montana convective systems have forced a more southern route, but at our speeds, it doesn't make a big difference in time.