I did AS and A2 maths last year, and now doing AS and A2 further this year, this is how I felt about the units I did/am doing:
C1 - boring, basically gcse
C2 - better, but I don't like trig or series so was still pretty dull
C3 - I hate graphs.
C4 - By far the best of the core units, I enjoy calculus a lot.
M1 - Pretty simple, but boring.
M2 - Hard. Hard but interesting.
FP1 - boring, but harder than C1 and C2, which makes it annoying. However better than C1 and C2.
FP2 - Some of it is actually quite fun (integration/differentiation atc.) but graph sketching and polar curve sketching is really dull and sometimes confusing, okay overall.
FP3 - Pretty much love it compared to everything else I've done, Diff equations and groups are great, vectors are a pain but interesting, and complex numbers are coolio if a bit difficult sometimes.
M3 - Why did I even pick this when I need to resit M2?
D1 - OH MY GOD THE MOST BORING UNIT EVER
S1 - Boring and I just hate stats.
I'm on OCR.
So yeah Further >>>>>>> Core!