Isshin is crazy strong. He beat Aizen (granted, regular Aizen, but this is still the Aizen who cut down several captains and visoreds in a single panel) with a flick of his finger. He also says that his Engetsu works similarly to Ichigo's Zangetsu and heavily implies that he achieved Final Getsuga--the thing Ichigo uses to beat Hogyoku Aizen--in the past. That is ridiculous strong. Plus, narrative wise, we have still haven't even seen his shikai, which he didn't need against Aizen, so that implies again that he is insanely powerful. He's just not much by rank, but we've already seen that rank means nothing, when people like Aizen, Soi-fon, and Dogface all held the same rank at the same time.