What is the highest rank in Heroes of the Storm?

I'm curious, can you enlighten me on the pinnacle of achievement in the realm of Heroes of the Storm? What is the esteemed title that denotes the utmost mastery and prowess among its players? Is it a rank that embodies the essence of strategy, teamwork, and individual skill, or is it something more elusive, a testament to dedication and perseverance? I'm eager to learn about this elite tier, and how one might aspire to attain it.