That's the hardest question of all.
I think we all know what the clear havens are. They range from the Cook Islands in the Pacific, which is not used much by our people, down to the Caribbean, where there are multiple islands. There are lots of zero- and very-low-tax places in the world.
In my experience, those places are not used so much by corporations. Another distinction that needs to be made here, by the way, is between individual avoidance or evasion and corporate avoidance or evasion. They're really two different subjects.
You're not going to find most of your public corporations steering huge amounts of capital through the Cayman Islands, because they fear the reputational risk among other things. But if you're going to get serious about havens, you have to at least ask yourself what you are going to do about Ireland, what you are going to do about Singapore, what you are going to do about Hong Kong, and what you are going to do about Luxembourg, because those jurisdictions are used by the multinationals and there's a lot of money coming out of the developed countries and going into those.
I think the starting place is to identify what you're talking about, and again, I think it probably is not a great idea for any one country to try to do it on its own. I really think that is a best practices kind of question.