I have worked in this field professionally, both as an environmental advocate (working on the charity side), and as a grantmaker (working on the side of the endowed foundations that have money to give to said charities). I'm not that familiar with Washington state, having done most of my work with international nonprofits.
But if you held a gun to my head and demanded an answer, I'd probably put my money on Conservation Northwest. This is for a few reasons:
The organization's mission and tactics are in close alignment with your stated goals (it connects patchworks of important land);
It utilizes SEVERAL different conservation strategies, that complement each other;
It's the perfect size for you, not so huge that it won't benefit from your contribution, but also not so tiny that it will no longer be around to receive your money when you croak;
I have met its executive director in the past and was extremely impressed;
It is financially supported by some particular foundations and foundation officers whom I know personally and whose judgment I REALLY trust. They do their homework and are very selective about which conservation orgs they will give their money to.
No affiliation, of course.
It's a good idea to go and meet with (or at least have a phone call with) the organization's director or fundraiser. You'll get a sense for its leadership and clarity of direction. In my experience, sometimes the most amazing organizations (in terms of true accomplishment) are understating their achievements in conservation, and others (often the really big charities, but I want to be careful of over-generalizing) are overstating. So you can't judge an organization by its website, its name recognition, etc.
There is no substitute for getting a personal feel for the organization via direct conversation. If you are considering putting them in your will, they owe you a conversation and should treat you with respect. See if they do that. Also, when you are including a charity in your will, sometimes it's important to include certain language, e.g. sometimes the charity is incorporated with a name that differs a little from the public name it uses. So, lots of reasons to check in with them.
It's all about leadership, having a very clear vision and a very strategic way to get there, and hiring smart staff. THAT is what creates conservation results in the field. It really has nothing to do with, say, how much is spent on programs versus admin.