This is very subjective and we can't answer this from your perspective as it depends what your strengths and weaknesses are. I did Biology and Chemistry and found chemistry much harder, however not everyone does.
Biology contains a lot of content and so is based more on memorisation. However the exams themselves are more about application and data handling rather than straight recall of content. E.g. they're more likely to give you questions based on experiments you've never seen and expect you to apply your knowledge to an unknown situation rather than ask you directly how the kidneys work (though these questions can come up). This, along with the sheer amount of content, is what makes it difficult however it is also very interesting and if you practice enough exam technique to know how the markschemes want you to answer them then you should be fine!
The other sciences tend to have slightly less content and are more based on maths, formulas, etc. and (from my experience) tend to have more right/wrong answers than biology where it is largely subjective.