I have Masked Man and Shisui and I would definitely say your priority should be Shisui > Kushina > Masked Man.

As much as I like Masked Man, he's kind of weak and as soon as you've used his mystery he basically becomes useless. There's also so many ninjas that provide immobile with their chase nowadays so you don't even need that from him. Also I don't think he fits in very well in the current meta.

Maybe he'll get more useful as people start using the new Jinchuriki ninjas, time will tell.