Which was the last country to abolish slavery?

As it's still going on, with people enslaved until the perpetrators are found out, I can't answer that question. If you mean the last country to cave in and make a law to abolish slavery (even if residents didn't obey) then that would be Mauritania.

But I can't check whether the practice is still going on despite the law. However, a presidential decree outlawed slavery in 1981. Pretty recent, isn't it?