What on earth? We know he's able to react to Aizen
Pushed Aizen to his shinigami limited
They were potrayed as equals in CQC
Isshin has enough reiatsu to send Aizen flying through building with his mere finger.
I'm sorry, but from what we have seen Isshin defeats Shunsui 10/10
Shunsui is one of the stronger captains, but characters like
Yamamoto/Aizen/Isshin/Urahara are a tier above him.
Not to mention his GT is insanley powerful.
Aizen and Isshin were far from being portrayed as equals. First of all Aizen wasn't even retaliating seriously.
I'm lazy to go digging for the links at the moment (because Kubo makes stupid titles that don't even correlate with the chapters), but all we've seen from Isshin is a finger flick (which I'm not saying is weak) and a Getsuga Tensho. They were both strong but what other skills do we know?
A single powerful attack (in this case, his GT) doesn't personify a Shinigami's powers as a whole. The brute force of Ichigo's Getsuga Tensho is also powerful but that doesn't say anything else about his other attributes. Things like speed, agility, reaction time, experience, knowledge, etc all factor into a Shinigami's battle prowess and their chances of winning a fight.
Renji, for example, is the only Lieutenant (revealed thus far) to have a Bankai, but that doesn't mean he's the strongest Lieutenant.
My point is, no matter how strong Isshin appears to be, the fact of the matter is we don't know what he can do. Note that in none of my posts did I state he was weak or that he would lose. All I'm saying is that it's pointless to judge people in a fight when we don't have full info on them.
On the flip side, Kyoraku is the same case. We know nothing of his Bankai. The only hint we have is that Ukitake said it was unsafe to use in an area where many people could see it
Lack of knowledge means we can't know the outcome of a fight. Take Ichigo for example. If we made a "Ichigo vs Byakuya" thread way back then before we knew what his Bankai was, then the majority of people would probably choose Byakuya. But out of the blue Ichigo's Bankai was revealed to be a tremendous speed boost - a
PERFECT counter for Byakuya.
When comparing two high level combatants it's not (just) sheer reiatsu or powerful attacks that win the fight. Obviously this doesn't apply to two opponents in completely different leagues, but in Isshin and Kyoraku's case I find it applicable. They aren't worlds apart.
It's the specific powers and abilities that can turn the tides of a battle.