The top-tier.
‘S’ tier may stand for “Special”, “Super”, or the Japanese word for “Exemplary” (秀, shū), and originates from the widespread use in Japanese culture of an ‘S’ grade for advertising and academic grading.
Bibliography
“Tier List.” 2022. Wikipedia, August. https://en.wikipedia.org/w/index.php?title=Tier_list&oldid=1104486656.