Not having tried this type before, I will consider that such an arbitrary and subjective question may have different answers, none of which have any real meaning to anyone but the answerer as no one could possibly have a viewpoint that accurately encapsulates all the possible answers and accounts for the breadth of human experience and interest.
Kidding aside, this is not really something that can be settled by a discussion of specific disciplines. For me, the hardest engineering I have done is where the consequences of failure are the highest on public safety. It really changes your mindset.