Everyone is different mate. I personally found MET hard, purely because there is more to read then the rest (with the notes the school supply).
However, the ATPL's in general are not particularly hard, its more a matter of quantity.
Really easy ones tend to be:
VFR Comms
IFR Comms
Pretty easy:
Mass and Bal
Human Perf
R Nav
Law
Ops
Require more thought:
Flt Plan
Instruments
Met
G Nav
Perf
PofF
AGK
The above is my opinion.
HB