If you have a window seat, you'll be guaranteed to have good mountain views regardless of which side you're on.

However, typically the best up-close view is of Mount Hood, which would likely be on your left. I say likely because I've never been on the LH flight into PDX. But that's my best guess. Some flights, the peak of Mount Hood (11,000 ft) is even with the plane and is seemingly just outside the window. You could literally see mountain climbers with the naked eye if there were any there.