My a-levels were six years ago, but ok. Hardest to easiest;
Psychology - loads of cases and random info to remember, and I didn't find it that interesting so I struggled to force myself to memorise it.
Chemistry - Big jump from GCSE, from what I remember. Not too difficult, though, really, just slightly harder than bio/maths.
Biology - Not really difficult either.
Maths - Easiest by far, because I found it the most enjoyable.