Where is the world's largest national park?

The North West Greenland National Park! The park takes up almost half of the island of Greenland at 972,000 km2 (375,000 sq mi). The park has 500 visitors or less a year and has no permanent residents although a few dozen hearty souls may spend the winter in the park. The low impact of humans makes the park a remarkable place to see. One of the highlights is the northern lights.

https://en.wikipedia.org/wiki/Northeast_Greenland_National_Park