That depends what the meaning of “is” is. No wait, that’s not right. That depends on what the meaning of “Largest” is. That’s better. Oh, and it matters what you consider a lake.

If you are OK with a lake containing salt water, then it’s the Caspian Sea, which is a landlocked inland sea.

If you don’t count the Caspian, then it’s Lake Superior. But only if you count “largest” as most surface area.

If largest means “holds the most water”, then it’s Lake Baikal in Siberia. Baikal is a fresh water lake, and holds more water than the 5 great lakes of the US combined.

As an unapologetic American Patriot, I prefer the definition where the USA wins. So Lake Superior it is.

Source: Planet Earth on the Discovery Channel, then This Site, This Site, and This Site for research.