Renji has bankai, which immediately puts him near the top. However, Renji's kido skills his fairly poor. The other lieutenants, Hisagi, Kira, Rangiku are more balanced fighters. While Hinamori excels in Kido but herself admitted that she isn't as strong as others, however she is a clever and is adept at battle tactics. So really, it depends on what you been my "strongest". If you mean pure reiatsu, well that hasn't been measured but Renji can up his base reiatsu with Bankai.