Minato. If Itachi started out in Susano'o I would say Itachi, but Itachi never starts out in Susano'o. It would drain his chakra and he knows it. If Minato had prep time it would be even more of a stomp because he could place his FTG Kunai all around the battlefield and just go nuts.
The way I see it happening though with no intel is Minato throwing a FTG Kunai at Itachi while either preping a Rasaengan, or grabbing another FTG Kunai. Itachi would dodge the Kunai of course, and when Minato sees that the FTG Kunai is slightly out of Itachi's peripheal vision, he would teleport to his Kunai and either
A) Lethaly Cut him with the extra FTG Kunai
B) Rasaengan Itachi
C) Brand him with a Seal he could teleport to Itachi later in the fight
If he branded Itachi with that seal, Minato still would have a very good chance at killing Itachi. The only way I see Itachi winning is if he catches Minato in a genjutsu, but Minato should know that the Uchiha's are very adept in genjutsu so I still think Minato wins.