The Saguaro cactus (Carnegiea gigantea) is widely considered to be the largest cactus in the world. They can reach heights of up to 60 feet (18 meters) and can weigh over 10 tons.

However, there's some debate on what constitutes the "biggest" cactus. If you're considering weight specifically, then the Giant Barrel Cactus (Ferocactus giganteus) might be a contender. These cacti can grow incredibly wide, reaching up to 13 feet (4 meters) in diameter and weighing several tons.

Ultimately, the "biggest" cactus depends on how you define "biggest" - by height, weight, or other factors.