Among 'The Evil Quartet', which one is considered the most important cause driving extinction of species?

Loss of biodiversity is one of the major concerns in the modern developmental era. There are four causes, called 'The Evil Quartet', which includes:

1. Habitat loss and fragmentation. This is the most important cause driving animals and plants to extinction. The most important examples of habitat loss come from tropical rain forests. When large habitats are broken up into small fragments due to various human activities, mammals and birds requiring large territories and certain animals with migratory habits are badly affected, leading to population declines.
2. Co-extinctions
3. Over-exploitation for economic gain
4. Alien species invasions.