Depends on what you're looking at...
Had the worst childhood?
By far Gaara. He was told his mother hated him, he thought his uncle hated him, his uncle was the closest person to him and attempted to annihilate him- he felt betrayed, he couldn't sleep for more than a few minutes at a time and therefore he had a legit excuse to be mentally unstable (unlike most Naruto characters

), he thought his father loathed him, his father sealed a bijuu inside him, he was alone from the start, his own siblings were afraid of him, all villagers feared him, he was raised to be a weapon and nothing more. But, people still might not list him at having it the worst because he's a Kazekage now, with people praising him and an entire village backing him up. His life is great now and he's young with a bright, wonderful future.
Had the worst life overall?
By far Itachi, at least as far as what happened to them. However, people might not list him because he could handle the burden and didn't snap like everyone else or sink into despair at any point in his life like all other characters have in the series- Uchiha or not (Kakashi, Obito, Madara, Sasuke, Gaara, Nagato, Kabuto,etc). He also chose it to an extent- he could have run away and let the war occur to save his own skin, but his decent morals wouldn't allow that so he did what he did. He's selfless to an insane degree. He was content with suffering if it meant other people were happy.
Who reacted the worst?
Some fans measure pain by taking what a character went through and greatly exagerrating it, just as the character they fanboy/fangirl over does for themselves. Sasuke and Madara win that, they throw constant pity parties for themselves, didn't have the maturity to let go of the past and move forward, and cause themselves pain when they don't have to. Looking at some comments and seeing how a few fans already exaggerated Sasuke's past by stating the same points in a different fashion as well as leaving out the sad points of other character's lives confirms that.
Even if Itachi takes all the blame for Sasuke's negative behaviour, as a fandom we know that isn't entirely true. The only things Itachi is reponsible for is using genjutsu on Sasuke...this hurt Itachi more considering that he was hurting who he loved and wanted to protect the most all while knowing that Sasuke hated him with every fiber of his being. Sasuke also saw the clan die by Itachi's hands...but he knew none of them bar his Aunt, Uncle, Mother, and Father...Itachi knew more and had to live the rest of his life in disgrace knowing what he did. He was 7 going on 8 at the time...Itachi was only 4 when he saw 1000s of people massacred in the 3rd Shinobi War. That's worse, also the fact that the sharingan is a measure of an Uchiha's pain and that Sasuke didn't even get all 3 tomoe until he was 13 whereas Itachi had 3-tomoe and was proficient in genjutsu by the time he was 8...this isn't a contest tbh. Then the rest of Sasuke's pain is a result of his own actions, not anyone else's. He wasn't forced to kill anyone, unlike Itachi was when he was given a mission to annihilate his loved ones and they'd be annihilated if he raised his hand against them or not. Now that sucks.
All of this depends upon your perspective. But it is blatantly obvious who Kishimoto thinks had the worst past and it's stated by a few characters vaguely, already in the manga. Most of the fandom realizes who it is, anyway.