There is no "best" .ipsw file. The "correct" or "best" .ispw file is the one that will run on your phone ... Put your phone in DFU mode and use iTunes to update the software. iTunes will locate the correct .ipsw for your phone and install iOS 9 on it.
If you want to run iOS 8.4.1, you can still do so for the time being, you can do so, but you have to find the correct version for your phone - and there is no jailbreak for iOS 8.4.1, so it's hardly worth doing, IMO. Look at the back of your phone to find out what the model number is (Axxxx) and find the .ipsw for iOS 8.4.1 for that model.