I will answer based on the data and information available from internet.

The best one should be Dyson, I'm talking in terms of their design and R&D development. But you will need to pay Dyson Tax together with the brand name.

The best alternative is coming from China side, they have their own R&D development and great precision manufacturing. It's either Dreame or Roborock.

Make no mistake that Dyson is the undisputed leader. However for value wise, I will go for Dreame.

*You can search and view any documentary regarding the listed company above. They are solid manufacturer with great idea n expertise. In terms of technical and design they are all at the world level.