This looks to have been pretty widely passed around so far, but it's a good link, so let's pass it around some more. What we have is the first annual Global Slavery Index. What the Global Slavery Index seeks to do is provide a worldwide estimate of the number of slaves currently existing in each country on the planet. Obviously, it's going to be a tough task to count slaves, as, especially these days, that's not exactly something slaveowners are going to be open about. They're either going to hide them or call them something other than slaves.
In order to make the count, they essentially combine the definitions of three treaties, none of which were universally accepted but all of which were widely accepted: the Slavery Convention, the Forced Labor Convention and the UN Trafficking Protocol. By their count, they have it at 29.8 million people currently enslaved around the world. That is roughly equivalent to the entire population of Malaysia.
Mauritania comes out ranking 1st- aka worst- in the index. That link goes into detail about just how bad it is; I'll leave you to that. The rest of the top 10:
8- Cote d''Ivoire
On the happy side of things, the United Kingdom, Ireland and Iceland share joint top honors at a tie for 160th. New Zealand comes in behind them at 159th, and then there's another logjam encompassing Austria, Belgium, Denmark, Finland, Greece, Luxembourg, Norway, Sweden and Switzerland. The full rankings are here.