Definitely the Dursleys. Snape is used to the world of magic and has faced far scarier things. The Dursleys, on the other hand, live in a normal, non - magical world. When Snape confronts them, they would be completely out of their element. They wouldn't understand his powers and the strange things he could do, which would make them extremely frightened.
The Dursleys would probably be more frightened. They are muggles and Snape is a powerful wizard. The idea of a strange man in black robes suddenly appearing and using magic would be terrifying for them.
The Dursleys would be more frightened. Snape has a certain air of mystery and power about him. He can perform spells and use magic that the Dursleys can't even comprehend. They have always been afraid of the abnormal, and Snape is the epitome of the abnormal in their eyes. Snape, while he might be cautious, has the upper hand in this situation. He knows what he's doing and what he's capable of, while the Dursleys are just ordinary muggles who are suddenly faced with a very intimidating wizard.
In this fanfiction, Snape might finally expose the Dursleys' ill - treatment of Harry. He could use his magic to intimidate them into treating Harry better. Maybe he'd discover some dark secrets about the Dursleys that they've been hiding all along.
Snape would likely be disdainful towards the Dursleys. He has little patience for Muggles in general, and the Dursleys' mean - spiritedness would not endear them to him. He might use his cold, sarcastic tone on them, leaving them confused and a bit frightened.
Perhaps Snape uses magic to create a distraction at the Dursleys' place. He could make strange noises or apparitions that scare the Dursleys, and while they are in a panic, he grabs Harry and disapparates. This would be a quick and efficient way for him to rescue Harry.
If Snape stays at the Dursleys in fanfiction, it could lead to some interesting character interactions. Snape might have a complex relationship with Dudley. He could try to teach Dudley a lesson about bullying, using his Slytherin - like cunning. Or perhaps he'd end up having some sort of begrudging respect for Vernon Dursley's determination to keep the wizarding world out of his family's life, while at the same time finding it completely absurd. And with Petunia, there could be a whole range of emotions, from her fear of him as a wizard to a possible shared understanding of their complex feelings towards Lily Potter.
Perhaps Snape would initially mistake the Dursleys for some sort of Muggles with a bit of dark magic in them due to their cruel nature. He could start investigating them, which would cause chaos in the Dursley household as they try to hide their secrets from a wizard like Snape.
The Dursleys might be left in a state of shock and confusion. Since they have been so mean to Harry, they could be worried that the wizarding world will come after them for revenge. But since they are Muggles, the wizarding world may just leave them be, but they will always live in fear of the unknown.
In the fanfic, Snape might use his knowledge of the Dursleys' house layout. He may enter the house unnoticed, find Harry, and then use a Portkey to quickly get them both out of there. He could also have some sort of charm or potion that he uses to counter any magic the Dursleys might have used to keep Harry there, like a spell - breaking potion. And he may have planned the whole rescue very carefully, choosing a time when the Dursleys are least likely to notice, like in the middle of the night.
After saving Harry, Snape may have to deal with a lot of mistrust from some people in Hogwarts who don't understand his motives. However, he might also start to form a closer bond with Harry, which could change the whole dynamic between them. This new relationship could lead to some unexpected alliances and enmities within the wizarding world. And if the Dursleys were to try and retaliate, Snape would have to constantly be on guard to protect Harry from any potential harm they could cause.
Perhaps Snape discovers some dark plot involving the Dursleys that could harm Harry more than he initially thought. The Dursleys were known to be cruel to Harry, and Snape might have found out that they were involved with some Death Eaters or dark magic users who planned to use Harry in a harmful way. So, he decides to rescue Harry to prevent this from happening.
Well, he could use his magic skills secretly. Maybe he casts a spell on the Dursleys to make them docile or distracted long enough to get Harry out. For example, he might use a Confundus Charm on them so they don't notice Harry leaving.