"The proportional index ranked Singapore, Korea, Qatar, Kuwait, Japan, Thailand, Bahrain, Malaysia, Philippines and Netherlands as having the highest proportional environmental impact, whereas Brazil, USA, China, Indonesia, Japan, Mexico, India, Russia, Australia and Peru had the highest absolute impact (i.e., total resource use, emissions and species threatened)."
(Figure source: PLoS ONE. Upper panel - proportional impact, lower panel - absolute impact.)
They claim that this study is robust, quantifiable, and looks only at the environmental impact, unlike previous rankings and data compilations.
More details from Bradshaw's personal blog, and the original paper.
(Thanks to Hann for heads-up)