I found Sets, Relations and Groups and Discrete Maths the easiest ones; they're nice and lovely and pure. (I hope they're still called that in the new syllabus?)
Statistics was OK, but kind of pointless and easy to lose marks at. Analysis and approximation (now called Series and differentiation) was good, better than statistics in any case, but quite hard (not in the actual exam though, that was surprisingly easy in May 2005) - though this one has changed the most in the new syllabus, and I don't know exactly what it's like now.
The absolute hardest option for me was Euclidean Geometry and Conic Sections. It's not offered any more for HL Maths, and it has been cut to only a tiny bit of its former content for Further Maths in the new syllabus.
I suppose statistics might be the best option for economics; for engineering, I'd go for sets, relations and groups. (It doesn't *look* very useful when you do it, but group theory has lots of applications. It's incredibly useful in chemistry and physics, and I suppose engineers do symmetries and things like that as well.)
Also series and differentiation is useful - differential equations are important, and series, while hard, are very common in the sciences.