Slavery was abolished throughout the Middle East by 1968, and is now officially illegal in every country in the world. However, depending on the precise definitions employed, there are still many millions effectively living in slavery on a list topped by India and followed, in order of magnitude, by China (2.9 million), Pakistan (2.1 million), Nigeria, Ethiopia, Russia, Thailand, Democratic Republic of Congo, Myanmar and Bangladesh, according to the anti-slavery Walk Free Foundation's 2014 report. In 2013 the US State Dept released a report into Human Trafficking which identified the worst offenders as being Russia, China, Uzbekistan with Cuba, Iran, North Korea, Sudan, Syria, and Zimbabwe. The list also included Algeria, Libya, Saudi Arabia and Kuwait but no list has identified Abu Dhabi, interestingly.
Even if we allow that the US Dept's outlook is distinctly political, Walk Free is independent and widely respected. Walk Free's report also allocates an indicator according to the strength of each Government's response to slavery and how they deal with it. The Netherlands comes out best, with the UK being 8th on the list.
Reports:
US State DeptWalk Free 2014 slavery list