The absolute best coconut cake I ever ate was at my Aunt Doris' house in Jackson, MS.; however, she did not make it. At the time she had an older black woman clean for her a couple days a week - I think her name was Marleta, or something like that. Every few weeks she would stay later and make them something special to eat, and she made this cake every so often. Was lucky enough to be there one of those times. I remember she made the cake from cake flour - no box for her - and the texture was fantastic. For the frosting she cooked water, sugar and vanilla, and then combined it with whipped egg whites , then covered it with coconut. Only got it that one time. Another time I was there she made pecan pie, which was also one of the best I've had.
As for store or restaurant coconut cake, the one that I remember being really good was at Morrison's Cafeteria in Gulfport, MS.