I researched Puerto Rico quite thoroughly a while ago. I think it ended up being 0% capital gains taxes all the way around.
The problem is, you still pay full capital gains taxes on anything you've purchased up to the point of becoming a resident. It's only the capital gains taxes you accrue after becoming a resident that are tax free.
Takes half-year+1day to establish residency. Not too onerous, for someone sitting on a pile of cap gains. Not by half.