From what we have seen Minato use so far' I've concluded that he isn't strong enough to defeat Itachi. Let's take a look at Minato's abilities. He has flying thunder god technique level 1 and 2, he has a normal rasengan, he has sage mode(I know some of you might argue but just remember when the pa sage frog said Naruto had surpassed his sage mode predecessors, both Jiraiya and Minato were shown. And also Jiraiya taught Minato frog summoning so there was no way he wouldn't have learned Sage Mode), Frog summoning, Minato has that large transport seal that he used to suck tbb. He is also super smart in battle. That's it! All he has. Now on to Itachi. He has both water and fire ninjutsu skill, he is super talented in Taijutsu as shown in his battle against Sasuke, he has the Mangekyou Sharingan that bestows him with Tsukuyomi, Amateratsu, Susano,o which has the Totsuka Blade and Yata Mirror, Izanagi and Izanami. He also has crow summonings and is super intelligent in battle as shown in his battle against Edo Nagato.
Now this is how Minato 'cannot' defeat Itachi: ftg is fast I agree but Itachi has amateratsu. After Itachi died against Sasuke his Amateratsu still went to look for the white snake(Orochimaru) and killed him. If that was done by a weak Itachi this shows that a healthy Itachi can move his amateratsu out of his line of sight. He could station amateratsu at every possible seal where Minato could appear. Moving on to Toad Summoning and Sage Mode. Itachi sealed a 'sannin's' 8 headed snake with the Totsuka Blade. If he could seal Orochimaru's snake why cann't he seal Minato's frogs. Sage Mode makes you strong and fast, but not faster than the speed of light. Amateratsu'll get him I'm sage mode. Rasengan is pointless against the Yata Mirror or even the normal walls of Susano'o. The only problem Itachi would have is the large transport seal but he could easily dodge that with his great speed as shown in his fight with Sasuke.
Now the ways Itachi could defeat Minato: There is no place in the manga that proves Minato is Immune to tsukuyomi let alone Itachi's MS tsukuyomi so don't say that tsukuyomi can't work on Minato. How Amateratsu would defeat Minato is stated above. The Totsuka blade isn't fast enough to seal Minato when he's using ftg or is in sage mode but that's why he has Amateratsu. And if 'all' these don't work Itachi could sacrifice en eye for Izanami and keep on replaying an event in Minato's head. Again I say that Itachi can defeat Minato based on what we have seen Minato do so far. And loojk at both their sacrifices. Minato sacrificed his life for his village when he could have used another seal to seal kyuubi. Kushina was even asking him why he didn't use another sealing jutsu. He wanted to die. How can u lleave your child all alone in this world? But still I commend his sacrifice, he saved konoha and was known as a hero. Now on to Itachi. Itachi killed his clan at just the age of thirteen. He lived knowing he would be regarded by the same village he saved as an outcast. Minato died knowing he would be regarded as a hero. Tell me, which is worse? Now please don't you Minato fanboys ever compare thesr two men. Because they aren't in the same League.