I'm in year 12 and I'm doing chemistry, physics, maths and further maths and have done biology and geology to A2 in year 11.
I think in order of favourite to least favourite:
Biology
Chemistry (nearly on par with biology)
Maths
Further maths (nearly on par with maths)
Geology
Physics
Chemistry is by far my favourite so far at AS. In unit 1, you learn about how everything you did at GCSE is wrong or over-simplified (don't worry, it's very interesting) and unit 2 extends the GCSE ideas of energetics, kinetics, equilibria, redox to a higher level and a fair bit of organic mechanisms at the end, with the organic being my favourite.
If your school structures it how mine structures it (AS maths and AS further maths in year 12), pure maths in my opinion is great as it is either peasy or really interesting. C1 and C2 should be a doddle after GCSE and should be units where you are not aiming for an A, but for close to full UMS. FP1 is slightly harder, but you do loads of new interesting stuff, such as complex numbers, matrices, graphs/inequalities, properties of polynomial roots and some stuff on proof and series. Mechanics is the hardest part in my opinion, as if you make one error, the whole thing goes pear-shaped, but just requires plenty of practice. Statistics is dull in places, but has its interesting bits and isn't particularly difficult.
Physics has its fascinating parts and its dreadful, dire, mind-numbingly boring parts - a real mixture in my opinion. Electricity is awful and shockingly dull, mechanics is dreadful (like an over-simplified version of maths M1 where you have to try to rote-learn equations for different situations, but FM helps with this), materials is so-so, particles and quantum phenomena are VERY interesting and waves are interesting.
I think to answer your question more directly, it really depends on the individual as to their difficulty - it depends quite a lot on how interesting you find them, but my brain is quite memory/explanation/process-orientated, so this situation may be different for you.
Hope this helps