I'm nominating this one, along the side of a building that fronts Westlake in South Lake Union. For me, it almost has everything: Humor, details, beauty, nonsense, outer space. It's a free gift public art installation, and it's better than half of what we pay to commission officially.
I don't know whether it was put up piecemeal or by a team of artists working together, and I don't know how long it's been up. Closeups on the jump.