i had an argument with some people that if madara and itachi fight, madara will ultimately win. other people think itachi will win. i dont why people make itachi so strong.
No, that's a bit of a joke. In terms of raw power and capability there's no way Itachi could win in a battle against Madara. A few reasons:
1) Itachi when he was live was sick and not at full power. We can ask the question about a hypothetical "Itachi at his prime", but we don't know if such a thing ever existed. We don't know when Itachi got sick, or how powerful he was before that. He was still young, and we have no idea how powerful he would have gotten if he hadn't been sick.
2) Itachi was going blind. He had MS, and that was eating away at his vision. We know in the fight with Sasuke that his vision was very blurry. We also saw using his last Susanoo that his eyes were starting to go dark (whether that's an indication of him going blind or it's an oversight of the illustrators is another question). We also know that Sasuke's eyes "lost the light" after only three big battles: Against Killer Bee, Against the Kage, and against Danzo. Assuming Itachi and Sasuke's eyes have about the same tolerance, that implies that Itachi hadn't used his MS techniques much in the past and wasn't able to use them much in the future.
IF we ignore the reality of Itachi's character: Sick and almost blind from overuse of MS techniques, and IF we invent some kind of hypothetical Itachi who was at an unspecified optimum power level I still can't believe that Itachi would defeat Madara in a fight. Three more things to consider about Madara:
1) Madara's chakra was described as being unusually strong among the Uchiha. Itachi was described as being talented, but Madara was described as having unusually strong chakra (Itachi or Sasuke were never described like this). His powers upon revival have clearly been enhanced by Kabuto's medical experiments, but when he was still alive he was still recognized as being much more powerful than normal Uchiha.
2) Madara Not only has EMS (so he does not have to worry about blindness) but he also has unlocked the Rinnegan. He can theoretically use all the same techniques as Itachi (Tsukuyomi, Amaterasu and Susanoo, though we've never seen him use the first two) but can also use all the Rinnegan techniques as well. Can the Rinnegan's absorption ability absorb Amaterasu? Can his Meteor ability peirce Itachi's Susanoo? Can Madara use the 6-paths ability that Nagato used, or summon the Gedo Mazo, or resurrect people from the dead? We don't know the answers to these questions, which may turn out to be extremely important.
3) Madara was an expert user of the sharingan. He knew enough about it to unlock his MS, gain his EMS, and then unlock the Rinnegan. He also commanded the Uchiha during a time of endless war, when Uchiha warriors would have been studying the use of their eyes, and when people would have known about (and use) Izanagi and Izanami. Madara knows everything about the sharingan that Itachi knows, and probably more.
4) Madara is clearly very smart, he's created a complex plan for world domination which has taken decades to complete, involves the manipulation of entire generations of people, involves acquiring and unlocking legendary powers that most people don't know about, and is so far heading towards successful completion. Itachi is smart too, but can we say that Itachi is smarter than Madara? If so, is he so much smarter that it would make a difference in the fight?
I like Itachi a lot. I think his character is a little bit weird and I think Kishi could have done a better job writing his back story, but I do like the character a lot. However, he doesn't compare to Madara in a 1 on 1 fight. He simply doesn't. Now, if I had to put together a team of several shinobi to defeat Madara, I might include Itachi because his brains would help with planning and strategy, and his knowledge of the sharingan's secrets would be helpful, but Itachi cannot defeat Madara on his own. Sorry, it just can't happen.