You can find out by checking out the Happy Planet Index (HPI). The Happy Planet Index measures what matters: sustainable wellbeing for all. It tells us how well nations are doing at achieving long, happy, sustainable lives. Apparently, the happiest nation in 2019 was Costa Rica, with the UK coming in at No 14.
The data is based on the country's life expectancy, well-being and ecological footprint.
The website is full of fascinating insights - including a tool to help you measure your own personal HPI!