The closest to me AFAIK is Sealand, but I’d rather not, tbh. I do actually have a passport from Waveland, declared as part of a Greenpeace campaign some years back and based on Rockall, but also not too appealing as a long-term residence.
At one site that I lived and worked on for several years, we discussed declaring unilateral independence on several occasions. It was a shingle spit nature reserve and seemed a promising location, but we never did. Well, not so far.
Overall, the Free Borough of Llanrwst looks a good bet. I have been there and definitely enjoy the area.