C1 is the easiest, it's similar to GCSE but obviously more advanced. C2 is harder, yes, but manageable.
I have only done S1 (I have done AS only, not planning on doing A2) and in my opinion S1 isn't too bad. It's just a case of knowing the formulae and doing the past papers, as often the questions are the same each year with different numbers. So i would say chose S1, but if you like physics and you're planning on doing it at A level, M1 would be a useful choice because some topics will come up in physics.
But if you do the whole A level you will have to chose 2 of them anyway