Those Tasmanian trees are the tallest "hardwood" trees. The Sequoias and Redwoods are softwoods. The tallest tree now standing is a coastal redwood in Calif at 111.5 m. There was a taller Douglas fir in Canada but that is not longer standing. The Tasmanian trees at about 96 m, so they are tall. I never knew of them until your post.
Khortose is right that there is a humongus fungus among us. It almost sounds like something out of a science fiction flick.
Bob was right about the blue whale being the largest mammal but also the largest animal to have ever lived on earth.
The great barrier reef is indeed the largest living organism but comprised of many separate species.
The plant mentioned by GB could be the Aspen (Pando) Grove in Colorado/USA which is about 100 acres and all the trees are connected by their root system. The weight of this grove is estimated as 6000 + tons so likely the heaviest living organism and the oldest at 80,000 years. There are also creosote bush colonies in the Mojave Dessert that are 12,000 years old.
No one mentioned the Giant Sequoia trees in California which may weigh 1000 tons and are 1500 + years old which gets my vote as opposed to some creepy mushroom that hides underground.
