It's difficult to definitively say which floodplain is the largest because the concept of "largest" can be interpreted in different ways. Here are some options depending on the criteria:

* By area: The Amazon River floodplain is often cited as the largest in the world, with an estimated area of over 1 million square kilometers (386,000 square miles). This vast floodplain is formed by the Amazon River and its tributaries, and it's incredibly important for biodiversity and ecosystem services.

* By volume of water: The Ganges-Brahmaputra Delta in Bangladesh and India is considered the largest delta in the world, with an estimated area of 105,000 square kilometers (41,000 square miles). While not technically a floodplain, it's a low-lying area heavily influenced by riverine processes and experiences regular flooding.

* By historical significance: The Mesopotamian floodplain between the Tigris and Euphrates rivers is arguably one of the most historically significant floodplains. It played a pivotal role in the development of ancient civilizations like Sumer, Akkad, Babylon, and Assyria.

Ultimately, the answer to "which floodplain is the largest" depends on how you define "largest." The Amazon River floodplain is arguably the largest by area, while the Ganges-Brahmaputra Delta is the largest by volume of water. The Mesopotamian floodplain holds a special place for its historical significance.