The non-profit sustainable development organisation placed the city ahead of Brighton, Plymouth, Newcastle and Cardiff. Edinburgh, Sheffield, Leicester, Nottingham and London make up the rest of the top ten.
Bristol gained top spot through an increase in its recycling and composting rates as well as high scores for water quality, waste collection and green spaces. But the forum noted repeated criticism about the city's public transport.
The index's 13 indicators include air quality, ecological footprint, education, unemployment and authorities' commitment to climate change. However, the forum said even the UK's best performers cannot match international leaders such as Swedish capital Stockholm, Portland in the USA and Curitiba in Brazil.