Potions. Not necessarily because of Snape, although I’m certain he wouldn’t have liked me as a student (too Hermione, determined to be a teacher’s pet, which must have been hugely annoying for all of my Muggle teachers) – but because it’s too much of a science.
I wasn’t very good at Sciences in school. I like biology, as long as we didn’t cut anything up (massively squeamish) but chemistry and physics were too difficult. I didn’t like them. Or have the patience for them. And I feel like Potions would be a bit like physics and chemistry put together, so I can tell you know it would definitely be my least favourite subject.*
*except Arithmancy, but I wouldn’t have even chosen that.