I thought biology was the easiest: you have to remember a LOT of definitions and processes like Nick said, but you can memorize things without really understanding them. Physics is fine in mathematics, since you can include calculators in the exam; however, most formulas must be memorized. There are science classes for non-STEM students, geology and physics are the easiest. Biology is memorizing many Latin names and chemistry labs can get a GPA.
You should definitely devote yourself to geology if that is the class you feel most comfortable with. Science subjects such as Biology, Chemistry and Physics (along with any combined version) are also some of the easiest subjects to pass in GCSE. A key difference between Chemistry and Physics is the increase in the amount of mathematics needed to succeed in GCSE Physics, something that may explain the difference in difficulty between the two subjects. All GCSE students are looking for the easiest GCSE so they can do their best with as little work as possible.
Unlike Physics and Chemistry, the GCSE for Biology tends to be more application-based, as it requires you to apply your knowledge to several scenarios. This may be due to lower grade limits or the fact that many students who are already fluent in foreign languages choose to take them as an easy GCSE course.