I would go with Seattle, even though I don't partake in the weed. It's a city with a relatively low population, compared to other major cities. You'll have to be willing to deal with rain a good deal, but I actually enjoy rainy days. Denver is a great place as well. My family lives in that area and there is a ton of stuff to do in the surrounding area. Burning hot deserts and great skiing in Denver. Go where your heart desires though.
Yes, Seattle has a low population, but Denver's population is even smaller, just barely though. Seattle is the 22nd most populated city in the US followed by Denver at 23.