To study at school? If you’re good with numbers, I’d say physics. If you’re better with words, probably biology because, at least at school, biology is pretty descriptive and you can explain processes without necessarily having to use mathematics to describe those processes.
Once you go beyond school and do research as a career, I think biology can be quite hard because it’s often quite complex. In ecology, for example, it’s really difficult to explain why some species become rarer. Is it because the habitat has become smaller? Or because there are more predators? Or is it related to global warming? There are so many things going on at the same time that finding a clear-cut answer can prove almost impossible.
While it’s true that some sciences are more technical than others (for example theoretical physics requires a thorough understanding of mathematics, which some people – myself included! – would find difficult) when you get down to it all scientists are trying to find patterns in nature by observing the natural world, which is always a difficult thing.
Biologists, for example, might be able to avoid some of the maths, by they have to get to grips with the enormous complexity of living organisms. Being a chemist I generally have to get to know what molecules are up to – but obviously I can’t see them and they won’t just tell me what they look like (well easily anyway!).
I don’t think any science is easier than others- it just depends what it is that you’re good at. Some people find physics really easy- whilst for me, it’s not really my thing. I really loved chemistry in sixth form, but I was much better at biology and found it much easier.
I think, as Tobias said, if you’re good at maths then you’ll probably find physics easier than someone who isn’t. I was always much better at English than maths, which is probably why I found biology much easier. Chemistry, however, I wasn’t so bad at because I don’t mind the sort of maths in chemistry (at least the sort of stuff I still use day to day now like making solutions etc).