Which is the biggest root of the world?

It may depend on what you're looking for.

Notes indicate that the Monterey Cypress often will have a very large taproot.

The Sequoias are considered the largest trees in the world, but they tend to have a fairly shallow rootsystem. While the Root Plate on the Sequoias are huge, the rest of the root system may appear disproportionally small for the size of the tree.

Mangrove trees can have pretty amazing root systems.

I'm guessing the overall largest roots will be found with the trees that have taproots, although some trees apparently have early tap roots which eventually die off.