I really liked linear algebra when I took it in college. For whatever reason, I found it made sense, and corresponded well with my natural mathematical talents.
I spent a fair amount of time dealing with formal logic. The concept is interesting enough, although I don't think it's something I would like to deal with all the time - it gets rather dry beyond a certain point. But it does extend to real-life logical reasoning as well, which is a useful application.
On the not-so-great side, I never liked trigonometry very much. I think memorizing the identities, and subsequent confusion between them when I didn't use them for long periods of time, was a significant part of it - why an identity would be the square root of two over two didn't really click. The other part of it that never really clicked is the meaning of cosine and sine. From an academic standpoint of identifying which parts of a triangle constituted each, I could give a correct answer, but I had no idea why they were called cosine and sine. So again, if I went a long time without using them, I'd get them confused. Tangent was slightly less bad, not because I knew where the name came from etymologically, but because the word was enough different to remember more reliably.
I've also had negative experiences with calculus. It was so-so in high school - I didn't like it as much as algebra, statistics, or discrete math, but the places where it was useful in physics were kind of cool. In college, after taking a semester without calculus to do linear algebra, coming back to calculus was painful. A poor professor didn't help, but I think there were other factors such as not being able to relate to the applications since I wasn't taking physics or other science courses that used it at the time. The memorization issue likely also factored in - memorizing things such as Stokes' Theorem without really understanding why they worked wasn't something that played to my strengths.
Ultimately, I switched from a planned major in math to majoring in computer science midway through college. My grades were consistently high in compsci, and consistently decreasing in math, which was a factor. But I think a lot of the reason why I was doing better, and really was more interested in compsci as well, was that I could relate to the applications of what I was studying. Even outside of the areas mentioned above, I was moving more into theoretical math as I took higher-level courses, and relating to it could often be difficult. As I got farther into college, I was looking forward to making a practical impact, so the difficulty in answering the question of, "what would I do with theoretical mathematics outside of academia?" discouraged me from studying it farther. I think a more hybrid approach between mathematics and either hard sciences or compsci, while still teaching the "why" of how things work, could have kept me more interested. As an example, coding theory was an interesting, if challenging, area to study in the math department, but I probably would have enjoyed it a lot more and got more from it if we'd had some compsci exercises where implemented some of the algorithms we were studying in computer code.
All in all, mathematics went from my favorite or tied for it every year through 8th grade, to middle-of-the-road in high school, to an area I no longer had much interest in by the end of college. Better teaching elsewhere was part of it (the English/literature department was way better in college than high school, for example), but a general difficulty seeing how it translated to the non-academic world was a large part of it as well. Academics for academics sake was good enough for me through a good part of high school, but by the end of college, it wasn't anymore.
