Hardest: STEP III (technically OCR, IIRC) Easiest: Edexcel Maths S1
Not an A-level though, although I suppose it gets thrown into the same sort of category. Although I think it would be better if normal A-level maths pushed towards STEP sort of style.
Hardest: AQA A2 Law Unit 4 Medium: AQA AS History Unit 1(making of modern Britain) (was a retake so that's why it was not that bad lol) Easiest: OCR A2 Sociology Unit 3