What is the highest quality diamond grade?

Could you kindly elaborate on what constitutes the highest quality diamond grade? Is it primarily determined by the diamond's clarity, cut, color, or carat weight? Are there any industry-standard grading systems or certifications that one should be aware of when assessing the quality of a diamond? Understanding the intricacies of diamond grading can be quite intricate, so I'm curious to gain a deeper insight into the criteria that define the pinnacle of diamond quality.