The largest organism by mass is a quaking aspen colony in Utah named 'Pando' weighing over 6,000 metric tons. It's a clonal colony sharing a single root system. While Armillaria (honey fungus) covers more area in Oregon, aspen has greater biomass. Blue whales are the largest individual animals but far smaller than these clonal colonies. 'Mushroom' and 'amoeba' are too vague.