I have no idea how this event stands up when compared to others, but the 1992 Earth Summit, held in Rio de Janeiro could be a possibility.

The first global conference on the environment since the collapse of the Soviet Union and the end of the Cold War, this was a time of great optimism and confidence. As such, the conference was attended by 108 heads of state and 172 countries in total. The summit was relatively successful, with delegates agreeing upon a number of initiatives to begin to combat climate change.

Sources: UN.org, "UN Conference on Environment and Development (1992)

Gary Haq and Alistair Paul, "Global Environmental Governance" (2012)