Turn 1 is hard to carry speed through, but not hard simply to take. Turn 7 is definitely the hardest turn on the track for me, in every version of it I've ever played (never driven there in reality) - although the Parabolica can be a real killer if you enter it wrong, because you can lose half a second with a sloppy line through there. Or gain 3 tenths, like Vettel did pretty much every lap; he was the same speed or slower than Alonso everywhere else, but he always got back a big lump there. Since I doubt Alonso was taking a rubbish line into the curve consistently, I put that down to the high-speed aerodynamics of the Red Bull.
EDIT: I don't usually think of 4 or 5 as being hard - that's where I usually gain most of my lap time over the AI racers, particularly if I can get a nice exit off of 5. But then I've always had issues with chicanes and not so much with proper corners.