Minato mid difficulty. When you are too fast for the sharingan, it's impossible to cast a genjutsu(said by Raikage), so Minato who is even faster it will be even much more difficult. Only chance to cast a genjutsu for Itachi is to wait when Minato is immobile, but i'm sure Minato has knowledge about Sharingan since he knew exactly that Tobi couldn't control the Kyubi for too long for example. That means he learned enough about the hypnotic power of the sharingan to do such a stupid error especially when you know his genius(both of them are genius though)
Minato is much more faster than him and he only needs to touch you one time(putting his marking on you) in close combat to be able to finish the fight anytime he wants thanks to FTG. Itachi would most likely have knowledge about that since he was born after Minato, so he knows that his only chance is to avoid close combat but against someone as fast as Minato, he would be forced to use Susanoo to protect himself and he doesn't have enough stamina to keep up Susanoo for too long. Even for guys like Sasuke or Madara who have much more chakra, we've even seen that in most of their fights they don't use Susanoo all the time cuz of side effects of this jutsu. That's why if he struggles even a bit, Minato could take this opportunity to finish the fight
Minato could even finish the fight much more quicker if he manages to push Itachi into the radius of his FTG jutsu and the radius of this jutsu is great something Itachi most likely doesn't possibly know. So he could fail to notice when it's already over, and that could help Minato to teleport either attacks direct to him(Itachi) with his space time barrier or teleport himself inside Susanoo and finish him
Another possibility could be to destroy/immobilise Susanoo and the user by using Gamabunta(exactly like how he did with the Kyubi), and take advantage smh. There is also another possibility but i think this is already enough to explain why i see Minato winning most likely
