I'm very good at science so I didn't find any of mine particularly hard, but nevertheless:
Physics was probably the most difficult because I had to teach it to myself (teacher was beyond useless) and the CCEA exam board always write really awkward questions.
Biology was next because the AQA papers are filled with idiotic questions with awkward marks schemes
Chemistry was the second easiest because I love Chemistry and have a natural aptitude for it.
Maths was the easiest because it's usually the same thing every year.
Strangely, my lowest mark was maths, but that was solely to do with one paper (C3)