Dinosaurs! No. Blue whales! No? Pandas? Ha. It really depends on what you consider a thing and what's not a thing. And what's actually living and what's not traditionally considered living but is. And what's on Earth and what's inside Earth. Because if we're counting any sort of organisms, it could very well be mushrooms and trees that take the world's biggest title. [MinuteEarth]
I didn't watch the video yet, but i'm pretty sure General Sherman is the largest living thing on earth. But the largest living title belongs to a fungus that's underground in one of the rainy states.
On that note, if you haven't visited the Sequoia National Park, then you're missing out on some larger than life trees.