It depends on what you want in an open world.

Zelda BOTW is the best for how you interact with the environment and the sheer level of freedom in terms of handling the main quest. The Witcher 3 is the best in terms of the people and creatures populating the world which you can interact with. In terms of the visual design of the world I'd say Xenoblade Chronicles X's Mira is still the best open world.