In terms of time consumption, Systems & Control and Product Design were killers. They weren't particularly hard (I got 100% in my coursework for both subjects) but all the work you had to put in for the PowerPoints and the actual projects was horrifying. I spent about two weeks trying to glue down supports for mechanisms in my Systems project because the glue wasn't strong enough... or maybe I wasn't good at using glue. Hmm.
Note to current GCSE DT students: If in doubt, evaluate your way out. If you explain everything that went wrong in your projects - and by everything I mean every tiny detail - it's highly likely that you'll get 100% in your coursework. That means you only have to get about 75% in your exams to get an A* overall, which is a great relief.
In terms of knowledge? I'd probably have to say triple Science. There is so much to learn, yet in the actual exams you get quizzed on about... what, 30 maybe 40% of that? That's why I prefer Maths exams - they ask you about a much broader range so that you're guaranteed to know enough to get a fair few marks. That being said, I hate how with Edexcel Maths you have two exams in the space of a few days and then it's all over.
That's just my experience though. From what I've heard, languages can be pretty hard but I guess it depends on your teacher and their teaching style, perhaps more so than with other subjects.