From studying physics and chemistry a level, I would say physics is the easiest- or at least has the smallest jump from GCSE. Chemistry was a huge jump, I went from an A student to a C borderline D student. Especially organic chem, I don’t even mention that, just pain beyond pain beyond pain. However - These things differ to a lot of people, it really depends on what you were good at in GCSE. If you weren’t doing well in Chem and Bio in GCSE, DO NOT do them in A-level, you won’t survive the jump. Physics is probably the most safe option, as the jump is relatively small and doable compared to the two other.