The firm said the US's shift in policy since November 2016 has "resulted in the demise of the Clean Power Plan, which has made renewable investors more nervous about possible reductions to the investment tax credit and production tax credit".
The UK has re-entered the top ten, thanks in part to the impending contracts for difference (CfD) auction.
"While the UK environment for renewables is more settled than in recent years, which saw subsidy cuts, there is little clarity around the landscape after Brexit. Issues around renewable energy targets, subsidies and connections with mainland power markets are unlikely to be resolved before Brexit," the report said.
The UK's gain is Brazil's loss in the rankings, with the South American country slipping from 8th to 13th after it cancelled a wind and solar auction in December.
Another big fall saw South Africa's ranking descend to 19th from 9th position in October. The country suffered a credit rating downgrade to "junk" status in April, and the stand-off with Eskom harmed its attractiveness.
New entrants to the rankings include Kazakhstan (37th), Panama (38th) and the Dominican Republic (39th).
Kazakhstan benefited from a €200 million loan from the European Bank for Reconstruction and Development (EBRD) to improve the country's renewable and transmission systems.
EY said Caribbean and Central American countries were following Mexico's (9th in ranking) example to combat energy security risks and meet demand.
One of the strongest improvers on EY's rankings was Jordan, which climbed from 33rd to 21st, thanks to strong solar and wind resources. "There are plans to award 300MW by the end of this year: four solar and two wind plants, each at 50MW helping to edge toward a 2020 target of 1.8GW," EY said in its report.
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