Melocactus ernestii:

Once a Melocactus reaches maturity, its stem will stop growing and its cephalium (that bristle-coated structure on the apex of the plant) begins to grow, and will continue growing for the remainder of it’s life. This makes mature cephaliated Melocacti highly prized amongst rare cactus collectors. Melocactus ernestii wins the designation of "longest spines" amongst the genus.