Utah

on earth is a fungus.

Covering 2,385 acres of Malheur National Forest, Oregon, it’s our largest organism, and from the way the fungus has been growing, it may also be our oldest organism. Humongous is estimated to be around 2,400 years – 8,650 years old.

[link:http://www.nationalgeographic.com.au/nature/the-worlds-largest-living-organism.aspx]

Not to take away from the impotance of this article. But just for the information.

There's also a kelp field that some consider to be the largest single living organism but I guess it's too big to measure for sure.

On edit: there are a lot of conflicting statements on the internet about largest and oldest even about the same organism. For instance the Pando has ages all over the place. Not sure who's right.