For most of the Harry Potter characters, even the adults, it’s very obvious which Hogwarts house they were in when they were younger. For a few, however, we never find out, and while J.K. Rowling has given insight into some of these on Pottermore, others remain mysteries. So here are my thoughts:
It would be easy to assume that Aberforth Dumbledore was a Gryffindor to match his brother Albus, and he does have the bravery and impulsiveness associated with Gryffindor. However, he expresses a feeling of being overlooked and overshadowed by his brother, which likely would have grown stronger if he were in a different house, perhaps one that did not get as much respect. He is also extremely loyal, particularly to Arianna, and seems far more grounded than Albus, who was always full of brilliant ideas and big dreams. He does not mind working hard without any personal gain or ambition, first as Arianna’s caretaker and later at the Hog’s Head pub. While many others might have taken advantage of a famous relative to become famous themselves, Aberforth seems to shy away from the spotlight and prefer a simple life. Therefore, I think there’s a strong argument to be made for putting him in Hufflepuff.
Barty Crouch Sr. has all the traits of a textbook Slytherin. He is ruthless, ambitious, and willing to sacrifice others to preserve his reputation. However, he is also strongly opposed to dark magic, and Slytherin’s reputation as the darkest house would likely keep him away. I think the hat would have tried to persuade him, as it did with Harry, that Slytherin would be the best fit, but eventually given in and put him elsewhere. Where, you ask? I could see him as a very misguided version of either Gryffindor or Hufflepuff. On the one hand, he strongly values justice and law, in a very rigid way. He wants to see wrongdoers punished, and he’s dedicated his life to making sure they are. On the other hand, he’s not truly fair. He throws people in Azkaban without trials and secretly arranges to save his own Death Eater son despite publically disowning him at the trial. It takes a lot of bravery to stand up against evil, and he certainly makes a lot of enemies by doing so. Gryffindor might not be out of the question for him either.
Barty Crouch Jr., on the other hand, could only have been a Slytherin. He shared his father’s ruthless ambition but not his aversion to dark magic. Besides that, though, he was a master of trickery and disguise. He almost singlehandedly brought Voldemort back from the dead by manipulating a school competition, not by any means an easy task to pull off. He also managed to fool Dumbledore not once but twice: the first time as a young boy who Dumbledore seems to think may have been innocent of the crimes he was convicted of, and the second time disguising himself as one of Dumbledore’s old friends and colleagues, then spending a year teaching at Hogwarts undetected. The younger Barty Crouch was Slytherin to the core, which – come to think of it – was likely one of the factors playing into his strained relationship with his father. Not to mention that it might have given the elder Mr. Crouch a reason to believe that his son – who he’d certainly raised to hate dark magic and had previously been proud of – was in fact guilty, rather than simply in the wrong place at the wrong time.
Amelia Bones is surely a Hufflepuff like her niece. As one of the Wizengamot members presiding over Harry’s trial in Order of the Phoenix, she stands out among her peers as being level-headed and open to hearing Harry’s side of the story. She is there to hold a fair hearing, not simply to ensure Harry is expelled from school, which sets her apart from Fudge and Umbridge. This devotion to justice is exactly what made her a good Head of Magical Law Enforcement, and – unfortunately – a target for Voldemort. Names in Harry Potter often carry clues about the character as well, and Amelia comes from a Germanic root that means “work” – perfect for a hard-working Hufflepuff!
Rufus Scrimgeour might look like a Gryffindor at first sight, and he’s certainly brave. You’d have to be to spend most of your life as an Auror. However, my instincts are saying Slytherin. Scrimgeour does not crave power for its own sake, but he’s certainly convinced that he knows best how to fight Voldemort, and his methods are those of a Slytherin. His campaigns are, in fact, more about public perception than actual warfare. He wants the magical world to believe he’s accomplishing something, and if that means trying to bribe the Chosen One into being a Ministry puppet and sending a man who is almost certainly not a Death Eater to Azkaban, then so be it.
Mr. Ollivander is definitely a Ravenclaw, with his deep knowledge and understanding of wandlore. Harry mentions a couple of times that he is not entirely sure if he likes or trusts Mr. Ollivander, who seems more fascinated than horrified by the idea of Voldemort’s power. However, this is not because Ollivander is secretly a Voldemort sympathizer or because he craves that sort of power for himself. It’s an intellectual fascination. Ollivander has devoted his life to studying wands and magic, and Voldemort is an intriguing case study, if also a horrible person.