Hey, I do Further Maths, Maths, Physics, Chemistry.
This question is highly subjective any my only answer is that all the sciences at A Level are excruciating to most, but in different ways. A lot of my friends would say Chemistry, but I find Chemistry my easiest A Level (because I read unhealthily ahead in Y11 because I knew I wanted to do it), and Physics quite harder.
Anything STEM really FM, Maths, Physics is your main ticket, but you have FM which already opens a lot of doors in that field.