Author, do you mean best for running ICO or investing in ICO?
The answers will be different.
For running ICO are better countries with little or no regulation.
For investing in it are better countries where there is regulation because it'll be safer for investors to invest into checked ICOs.
I meant running an ICO
Also we're US residents, the country should have Double tax treaty with the US, so we won't have to pay double taxes from any potential profits on the project.