Maybe best masterpasword application is ink pencil and paper notebook and due to a hardware wallets they are safest but there alvays a risk becouse they must comunicate with outside world.
Depending on how much you're trying to store you could do a few other options - assuming it's bitcoin you're trying to work with here:
You could spend about $80 on a raspberry pi without Internet connection (the pi itself is only about $20, if you already have a keyboard, mouse, hdmi cable and spare micro sd card).
You could also buy a cheap laptop for about $100-200 and keep it offline for it's lifetime
(both these options would require a USB stick to facilitate data transmission - or Web cams with qr codes).
And yes there's also paper but I wouldn't call it secure..
If it isn't bitcoin you should try to enable 2fa everywhere too so you can use a code to unlock the account in case your password gets stolen - these are normally one time passwords used that change every minute and can be generated by a few different apps..