No question that cats, feral and otherwise (well, they are ALL feral to some degree) are hugely destructive to wildlife, particularly birds. And they have the unique ability to induce stupidity in humans.

But for sheer destructiveness of both habitat and wild creatures, from insects to albatrosses - and sometimes even humans - it would be hard to top the scorched-earth accomplishments of rats. With additional points for their superb abilities as disease vectors.

A good read is "Rat Island" by William Stolzenburg. It's a rather horrifying history of the destruction of the world's unique island wildlife by rats, cats, and other introduced vermin, as well as the war being waged against these invaders. And it documents the battle to retake our own Rat Island in the Aleutians.