It seems almost like Slytherin's reputation as a Hogwarts house will forever be scared by the fact it produced the most Dark wizards when compared to the other three houses. Not to mention its founder, Salazar Slytherin desired pure blood supremacy at the school, so much so that he kept a giant serpent to kill Muggle-born students. It does not help that his direct descendant the deadliest Dark wizard to have ever lived, Tom Riddle a.k.a Voldemort, wished to uphold this perverted ideology. Despite its dark history, Slytherin has produced good wizards. For instance:
1) Merlin
Yes. Apparently, writer JK Rowling made one of the most famous wizards of all time, Merlin, a member of Slytherin. In fact, in the Potter Universe, Merlin is so revered and respected that the Ministry for Magic awards something called The Order of Merlin to deserving wizards and witches to celebrate an outstanding achievement.
2) Severus Snape
This man may have started out as a Dark wizard, but he changed for the woman he loved. While for most pure blood supremacists, nothing else matters more than lineage. But Professor Snape, despite knowing that he can never be with the one he loves, sacrifices his own life to ensure the downfall of his former master so that goodness will prevail.
3) Regulus Black
Unlike his older brother Sirius, Regulus lived up to the family's legacy by believing in pure blood supremacy and eventually became a Death Eater at just sixteen. But after witnessing how his Dark Lord tortured Kreacher in order to secure the safekeeping of one of his Hocruxes, Regulus had a change of hearts and betrayed Voldemort. While most Death Eaters think human wizards such as themselves are supreme to all other living creatures, Regulus actually cared for the loyal house-elf who served his family for years. This proves Regulus was merely conforming to his upbringing and is not actually evil. During the Battle of Hogwarts, Kreacher, rallied the other elves working at the school to fight in honour of Regulus.
4) Andromeda Tonks
Like her cousin Sirius, Andromeda defied their family's pure-blood traditions in almost every way she could. She married a Muggle-born, something that pure blood Slytherins almost never do. Unlike her sisters Bellatrix Lestrange and Narcissa Malfoy who joined Voldemort's Death Eaters, Andromeda aligned with Dumbledore's Order of the Phoenix. Leading up to the Battle of Hofwarts, Andromeda and her husband were tortured by Death Eaters for information on Harry Potter and the Order but she resisted, proving her loyalty to her loved ones.
5) Horace Slughorn
Professor Slughorn is not exactly a model citizen, but compared to other Slytherins, he is surely way better. Yes, he displayed high favouritism with his Slug Club, but its members were chosen not based on lineage but rather their own merits and talents. As Potions Master, he was a much friendlier and motivating than his predecessor and former student, Snape. Although he loved aligning himself with powerful people, he knew where to draw the line. When he first learned Voldemort was actually Tom Riddle and that he might have created Hocruxes thanks to the information he shared with him, Slughorn felt deeply embarrassed and guilty, going as far as to hide away from public and even tampering with his own memory. Also, he showed he too has high morals when he battled his former favourite student, Voldemort during the Battle of Hogwarts.