I don't think you can drop the "A" while preserving the original meaning. Without it, an implicit "The" is assumed: "World's worst invasive weed" => "(The) Worl

I don't think you can drop the "A" while preserving the original meaning. Without it, an implicit "The" is assumed: "World's worst invasive weed" => "(The) World's worst invasive weed" Or at least that's how I interpret it as a native speaker!