The hard parts
In an industrial setting, the obvious plan is to seek out the easy work. You're more likely to get it done with less effort and then move on. The easy customer, the easy gig, the easy assembly line.
Today, though, it's the difficult work that's worth doing. It's worth doing because difficult work allows you to stand out, create value and become the one worth choosing.
Seek out the difficult, because you can. Because it's worth it.
[An aside for entrepreneurs and anyone starting a new project: if you can't describe the hard parts, how will you focus on them? And if there are no difficulties ahead, what makes you think your project is valuable? When I meet an entrepreneur, I always ask this question first--which part of your project is hard?]