score:5
I would avoid the coast. My experience is in California, where there is a well-known pattern of heavy fog and clouds near the coast in May and June (they call it "May gray" and "June gloom"). I don't know for sure, but I would expect similar conditions further north as well. However, often one can find clear skies just a few miles inland.
Deserts are much more likely to offer clear skies, especially in May. I don't know that there will be too much difference between your other three options.
Albuquerque has nearby mountains which might put you above any urban haze.
The Grand Canyon of course doesn't have any urban issues (though I understand there could be noticeable air pollution) but it is harder to get there (you have to drive) and there are not so many options for accommodations (unless you want to camp out).
Reno is not a very exciting place if you don't like to gamble, but it does have a lot of cheap hotel rooms. The nearby Sierra Nevada mountains could give you a remote spot for viewing (though they are also more likely to have weather), or you could head east into the desert (there are mountains that way also).