I like the thatch idea! ;-)
According to a variety of sources--for example this one--it's probably slate shingles, closely followed by concrete or clay shingles, with metal not far behind. In all three cases, barring natural catastrophes, you will have passed on before the roof wears out.
There are terra cotta (clay) roofs in Europe and the middle east that are hundreds of years old, but there are also people who maintain those roofs and occasionally replace broken shingles. Metal seems appealing in the Pacific Northwest, especially if you're in an area with extremely high snow loads, because the metal roof itself will be very strong, but weigh less than slate, clay or concrete. You can apply metal to a very steeply pitched roof that will shed snow more easily (reducing the load on your structure and on the roof itself), and it will have ridges extending down the slope of the roof that will help naturally segment the snow into sheets and provide a channel for it to slide on. And, of course, some people like the sound of rain on a metal roof.
There are architectural composite shingles that look more or less like traditional cedar or clay shingles, but they haven't been around long enough to know for sure how they perform over decades or more of exposure. Cedar shingles would really look nice, but you'll be replacing them in your old age.
"Traditional" asphalt shingles will probably cost between $0.80 and $1.50 per square foot unless you have a complicated roof design, which will drive the cost up. You might get away with doing a metal roof for three times the cost of asphalt and still have a roof that will outlive you. Slate will probably cost at least $10 per square foot, up to $40. Clay and concrete will fall somewhere in-between.