Hmmmmm I'd say ST6.
General was hard when I wrote it (as a 300 paper), not because the course material was hard, but because you never knew what the examiners wanted - eg when doing past papers you'd list certain things as the 'most important' and they'd list others, or your 'list' would be four detailed points and theirs 16 bullets, and the next time yours would be 16 bullets and theirs four detailed points!
ST6 has very 'buggy' notes which annoyed me no end, and not helped by having two books now all of which use different approaches and/or notation. Also the knowledge was skimmed over in places, and to the pure mathematician the heuristics would have given them a heart attack.....

And the examiner and ActEd seemed to have vastly different ideas of what the syllabus was actually after. It seems to be settling down a bit now, to be fair.