It depends what you're good at and what you enjoy doing. Some will say Chemistry is the hardest, and some will say the same about Physics and Biology. Think of it this way:
Physics - lots of maths, lots of equations, lots of theoretical stuff about electricity etc
Chemistry - a fair bit of maths, lots of stuff about atoms, electrons, etc., some stuff related to medicine I'd imagine
Biology - much more qualitative, so less maths, but more essays, aspects from Chemistry are involved in quite a bit of it, involves complex systems of things like cells, microorganisms, etc.