from what little we know about Isshin his amount of power seems to match that of a vaizard he did take out an arrancar without even releasing his shikai form and with his ability to make a shielding charm
Issin's secret power is that his reiatsu increases by 10% for every hair on his chest.
however, if you're referring to Grand Fisher, do take note that Grand Fisher was an incomplete arrancar, hence, at least, he'd be as strong as a gillian, and at most, a weak adjucas, in my opinion. in other words, he's weak and not a good source of measuring isshin's true capability.
Do I have to?
Well the fact that he's Ichigo's dad does hint at some unseen strenght...
hopefully, as strong as an arrancar, cuz now would be a perfect opportunity isshin steps in to fight with noitora.
Do I have to?
^I don't know about that. But Isshin's powers have been sealed for quite a while, so I wouldn't be sure we would ever see how strong he was when he was at his strongest.
^ unless there are some storyline flashbacks...
Isshin and Ishida's dad are probably powerful. In my opinion they probably can rival any shinigami Captains. hopefully there will be a flashback explaining their past.
^ Isshin, Ryuken and Masaki were exactly the same as Ichigo, Uryu and Oeihime now.
Y'know, children yonger generation following the steps of olders...
<feels sowwy for his Engrish>
There is a sig. You just can't see it.