Zaraki Kenpachi is the Captain of the 10th Division of Soul Society. He relies on brute strength and power. He is unstoppable with his Bankai.
Lille Barro is one of the royal guards of Yhwach. His attacks are unblockable, and desvastating. He is intangible and hard to deal with in his awakened form.
Askin is another one of Yhwach's Sternritters. His Schrift is one of the most powerful abilites among all the other Royal Guards.
Captain of Squad 6, Byakuya Kuchiki has been a powerhouse since the start of the Anime. His Bankai is powerful and unique, and he keeps getting stronger.
The current Head Captain, Shunsei is one of the strongest in the Soul Society. His Bankai is powerful enough to kill almost any opponent with ease,
The meticulous and powerful, Aizen doesn't need an introduction. Without even using his Bankai, he has subdued every opponent he has faced.
The leader of the Squad Zero, Ichibe is a force to be reckoned with! He can do almost anything with his brush and power over names.
By far the strongest Sternritter ,'The Miracle' is unbeatable. His schrift allows him to be essentially immortal and had to be killed Yhwach's power.
The original head captain, Old man Yamamoto was by far the strongest Soul Reaper. HIs Bankai could wipe out the Soul Society itself.
The father of all Quincies and "The Almighty" Yhwach. He was just too powerful. He had every ability of all the Sternritters and could see the future.