I think its a tie between Sirius and Remus.
Sirius was locked up in Azkaban for 12 years, he was able to live as a free though wanted man for perhaps eight months after that and lived the rest of his life as a virtual prisoner in the house he hated the most until he died trying to protect his godson and his friends. Harry has been wrong about lots of things, but I think he was especially wrong in this case, I think deep down Sirius actually wanted to die if only so he could be reunited with the person he truly loved, his best friend James.
Remus was a werewolf and because of that he was ostracized by the vast majority of wizard kind for most of his life. Although his parents loved him they died early in Remus’s life, one of his best friends was killed and he believed another had been as well, and his only living friend was accused of their murders and locked up in Azkaban. He was then reunited with that friend and was sadly able to spend only a year or so with him until he too passed away. He fell in love and got married, though I do question whether he genuinely loved Tonks, while he may have cared about her, he does express some resistance towards her that seems more than just related to his werewolf nature. Finally he has a beautiful son, but he is terrified that he will doom that child to a life of misery and considers leaving him before he is born only to be proven wrong when he is born, but is killed before he ever really gets the chance to know him.
Severus was a child deprived of the one thing human beings need the most of from the moment they’re born; love and acceptance. He was ostracized and bullied not just by Muggles (especially his abusive father), but fellow witches and wizards as well. He is able to find acceptance in a young girl he loved but she left him when he started practicing a very dark form of magic, befriended people who accepted him but who were also pure-blood supremacists, had developed and adopt racist and pure-blood beliefs, and finally decided to make it his goal to join that said racist and pure-blood terror group. Then he spent years trying to make up for the role he played in her death by protecting her son, even though he made no bones about hating and bullying the boy. Then he was forced to kill the man who was a mentor and perhaps his only friend to spare him a more painful death and was killed too in a most painful and agonizing way.
I’m not sure what people’s thoughts are on Peter’s suffering, but I’ve heard some comments that him being stuck in the form of a rat is bad, but was it though? Next to cockroaches, rats are the ultimate survivors, only slightly better looking and they’re actually kind of cool, don’t believe me? Check out Remy’s The Rat short from Pixar. Not only did he get to live with a wizarding family for most of his life, he got to live off Mrs. Weasley’s cooking, he ate whenever he wanted, slept whenever he wanted, and who the heck needs a T.V when you’ve got half a dozen wizarding children, two of whom were Fred and George causing general mayhem and mischief to keep himself entertained, he also got to go wherever he chose and even got to return to Hogwarts several times, and despite their objections otherwise both Percy and Ron had clearly cared about him, considering Percy broke his precious rules to get two other students to help look for Scabbers when he went missing and Ron stopped speaking to Hermione for a month when he believed her cat had killed him, and like Hermione said Peter had never once tried to hurt Harry as Scabbers, unless you count chewing his pillow as hurting someone, Sirius probably hurt Harry more in that year than Peter ever did in three, though not on purpose, and honestly I think if Sirius had never broken out of Azkaban, Peter would never have gone to join Voldemort, the only reason he did was because his “friends” forced him out of hiding. He wasn’t like Barty Crouch JR or Bellatrix Lestrange who were slavishly loyal.
James; I think had it easiest as his life was more or less perfect (and this may annoy some fans who don't like James). He had a loving family, he was a talented wizard, he was good at Quiddich, he had great friends and was considered good by most people despite he spoiled behavior. he got the girl eventually and he had a son, the one hiccup in his life being that his parents died when he was young. Yes; he also died young but his suffering was quick and painless, if he suffered at all in death we don’t know. It was his son and his friends who suffered the most in life, and that seems like a far worse fate to me.
In a way though all of the Maruaders and Snape(not Peter) got to live on in the next generation. In James Sirius Potter, in Albus Severus Potter, in Lily Luna Potter, and in Teddy Remus Lupin., so perhaps their fates will be better.
