Yamatomo with his bankai is the strongest character, he was killed by juckabach because he stealed his bankai, but he's still the strongest, why didn't he stomp all of aizen's army in one slash with bankai?
Because it would have been useless; Aizen already has him trapped in his Kyoka Suigetsu. Aizen would have taken a chhair, ordered an ice tea and let Yamamoto fight me some illusion copy of himself. And when he would have gotten bored, he would have executed Yama-ji
Because Kubo wanted to save Yamamoto's Bankai for this arc.