My FM modules were FP1, FP3, FP4, S1, D1 and M3 (did M1 and M2 with regular maths).
Obviously out of those S1 and D1 were the easiest, but of the modules that are purely FM modules i found FP1 the easiest. Then probably M3 was the next easiest - i always found mechanics the most boring and hard to learn on my own, but if i had a good step by step guide set out by a teacher (i had a fantastic mechanics teacher) then i found it okay. The hardest was probably FP4, i just found it really hard to get my head around, and it didnt help that we hadn't learnt the basic vector stuff in Core 4 yet :P FP3 is probably slighter easier than FP4 imo, but it is a lot of integration and quite easy to make silly mistakes i thought.
All this is reflected in my grades, where i got A's in everything except a high C in FP3 and a low C in FP4 (resulting in an A overall).