The phrase "enemy number" refers to someone who is considered the main or most important enemy. It suggests that person is the most significant threat or opponent. Full definition
Because saturated fat raised cholesterol, that suddenly became public enemy number one. (drhardick.com)
In recent years, though, they've become public health enemy number one, as evidence mounted that they contribute to heart disease, high cholesterol, and type 2 diabetes. (discovermagazine.com)
This makes the mutant HD gene enemy number one for those of us working to develop new therapies. (en.hdbuzz.net)