Samsung are worse as I've posted before...
Child labour exploitation - http://www.chinalaborwatch.org/report/90
Workers killing themselves - http://goodelectronics.org/news-en/samsung2019s-no-union-policy-claims-another-life
It's very, very difficult to accurately assess this stuff from thousands of miles away - we really don't know what is going on in those factories, many of which are shared by all the large brands we know about.
What we do know is Apple, the world's largest company by market cap., enjoys profit margins that are the envy of every other company in their industry. That should permit them to make greater efforts to address labour abuse issues than anyone else. Whilst Apple is booming Samsung's phone sales, for example, are collapsing. Which company is likely to try to make greater strides to protect workforces - the one with oodles of cash or the one seeking to cut costs? There's only one of these companies that have made concerted efforts to try and improve labour conditions. If they've slipped, you'd think this would push them to try harder still.
Additionally Tim Cook, who was recently voted FT's person of the year 2014, has made Apple a far softer and more empathetic company than was the case under Steve Jobs. If Apple were paying lip service to labour issues under Jobs (and I don't think they were), they certainly won't under Cook.